Quick Recipe Version (TL;DR)
Quick Ingredients
- 1 tablespoon (14 g) unsalted butter, for the baking dish
- 3 pounds (1.36 kg) Yukon Gold potatoes, weighed before peeling
- 2 cups (480 ml) heavy cream
- 1 cup (240 ml) whole milk
- 4 medium garlic cloves, finely minced
- 2 teaspoons fresh thyme leaves, chopped, plus 1 teaspoon leaves for finishing
- 1 teaspoon (6 g) fine sea salt
- 1/2 teaspoon freshly ground black pepper
- 8 ounces (225 g) Gruyère, freshly shredded, about 2 cups
- 2 ounces (55 g) Parmesan, finely grated, about 1/2 cup
Do This
- 1. Heat the oven to 375°F (190°C). Butter a 9-by-13-inch baking dish. Peel the potatoes and slice them 1/8 inch (3 mm) thick.
- 2. Gently heat the cream, milk, garlic, 2 teaspoons thyme, salt, and pepper for 5 minutes, until steaming with small bubbles around the edges.
- 3. Reserve 1/2 cup Gruyère and 1/4 cup Parmesan. Make three potato layers, adding one-third of the cream mixture to each. Divide the remaining cheese between the first two layers.
- 4. Cover tightly with foil and bake for 60 minutes, until the potatoes are almost tender.
- 5. Uncover, add the reserved cheese, and bake for 25 minutes, until deeply golden, bubbling, and completely tender.
- 6. Rest for 15 minutes. Sprinkle with the remaining 1 teaspoon thyme leaves and serve.
Why You’ll Love This Recipe
- Rich without being complicated: A gently heated garlic cream replaces the need for a separate flour-based sauce.
- A beautiful balance of cheeses: Gruyère melts into the layers, while Parmesan adds savory depth and a golden crust.
- Tender all the way through: Thin, evenly sliced potatoes and a covered baking stage help every layer cook properly.
- Perfect for sharing: This comforting side pairs beautifully with roast chicken, beef, ham, or a simple green salad.
Grocery List
- Produce: 3 pounds Yukon Gold potatoes, 1 head garlic, and 1 small bunch fresh thyme.
- Dairy: Unsalted butter, 1 pint heavy cream, 1 cup whole milk, 8 ounces Gruyère, and 2 ounces Parmesan.
- Pantry: Fine sea salt and black pepper.
Full Ingredients
For the Baking Dish and Potatoes
- 1 tablespoon (14 g) unsalted butter, softened
- 3 pounds (1.36 kg) Yukon Gold potatoes, weighed before peeling, then peeled and sliced 1/8 inch (3 mm) thick
For the Garlic-Thyme Cream
- 2 cups (480 ml) heavy cream
- 1 cup (240 ml) whole milk
- 4 medium garlic cloves, finely minced, about 12 g
- 2 teaspoons fresh thyme leaves, finely chopped
- 1 teaspoon (6 g) fine sea salt
- 1/2 teaspoon freshly ground black pepper
For the Cheese Layers and Finishing
- 8 ounces (225 g) Gruyère, freshly shredded, about 2 cups, divided
- 2 ounces (55 g) Parmesan, finely grated, about 1/2 cup, divided
- 1 teaspoon fresh thyme leaves, for finishing
Step-by-Step Instructions
Step 1: Prepare the oven, dish, and cheese
Heat the oven to 375°F (190°C) with a rack in the center. Generously coat the bottom and sides of a 9-by-13-inch baking dish with a capacity of at least 3 quarts with the softened butter. Set the dish on a rimmed baking sheet to catch any bubbling drips.
Shred the Gruyère and grate the Parmesan. Set aside 1/2 cup Gruyère and 1/4 cup Parmesan for the topping. Keep the remaining cheeses nearby for layering.
Step 2: Slice the potatoes evenly
Peel the potatoes and cut them into uniform 1/8-inch (3 mm) slices using a sharp knife or a mandoline fitted with its hand guard. Keep the slices together in a large bowl while you work, then assemble the gratin promptly to limit browning.
Do not rinse or soak the sliced potatoes. Their natural surface starch helps the cream thicken as the gratin bakes.
Step 3: Warm the garlic-thyme cream
Combine the heavy cream, whole milk, minced garlic, 2 teaspoons chopped thyme, salt, and pepper in a medium saucepan. Heat over medium-low heat for 5 minutes, stirring occasionally, until steaming with small bubbles appearing around the edges.
Do not let the mixture reach a vigorous boil. Remove it from the heat and stir well so the garlic and herbs are evenly distributed before layering.
Step 4: Build three creamy potato layers
Arrange one-third of the potatoes in the prepared dish, overlapping the slices slightly and spreading them into an even layer. Slowly pour over one-third of the warm cream mixture, including some of the garlic and thyme. Sprinkle with 3/4 cup Gruyère and 2 tablespoons Parmesan.
Repeat with another third of the potatoes, another third of the cream mixture, and the remaining 3/4 cup Gruyère and 2 tablespoons Parmesan from the layering cheeses.
Finish with the remaining potatoes and cream mixture. Press the potatoes down gently with the back of a spoon to settle the layers and encourage the cream into the gaps. Save the reserved topping cheeses for later.
Step 5: Cover and bake until almost tender
Cover the baking dish tightly with foil and bake for 60 minutes. Carefully lift one corner of the foil away from you to release the steam, then slide a thin knife into the center. The potatoes should be almost tender, with only slight resistance.
If the center still feels firm or crunchy, cover again and bake for 10 additional minutes before proceeding. Potato thickness and baking dish material can affect the timing.
Step 6: Add the topping and bake until golden
Remove the foil. Scatter the reserved 1/2 cup Gruyère and 1/4 cup Parmesan evenly over the surface. Bake uncovered for 25 minutes, until the top is deeply golden, the edges are bubbling, and a thin knife slides easily through the center without resistance.
If the cheese browns before the potatoes are completely tender, loosely tent the dish with foil and continue baking in 5-minute increments. If the potatoes are tender but you want a darker crust, broil on high for 1–2 minutes, watching constantly and using a broiler-safe dish.
Step 7: Rest, garnish, and serve
Transfer the gratin to a heatproof surface and let it rest, uncovered, for 15 minutes. This gives the bubbling cream time to settle and thicken around the potato layers.
Sprinkle with the remaining 1 teaspoon fresh thyme leaves. Serve in generous scoops or soft-edged squares, making sure every portion gets some of that golden cheese crust.
Pro Tips
- Even slices matter more than perfect slices: Consistent thickness helps the entire dish become tender at the same time. Always use a hand guard with a mandoline.
- Grate the cheese yourself: Freshly grated cheese generally melts more smoothly than pre-shredded varieties coated with anti-caking agents.
- Check the middle, not just the edges: The center is the last part to cook. A knife should pass through the layers easily before the gratin comes out of the oven.
- Use the specified salt: One teaspoon of fine sea salt weighs about 6 g; flaky or coarse salts measure differently by volume.
- Give it the full rest: The sauce will look loose straight from the oven. Resting helps it cling to the potatoes rather than run across the plate.
Variations
- Gruyère alternative: Substitute 8 ounces (225 g) Comté or fontina for the Gruyère. Keep the Parmesan for its savory, golden finish.
- Rosemary and thyme: Replace 1 teaspoon of the chopped thyme in the cream with 1 teaspoon finely chopped fresh rosemary. Keep the fresh thyme garnish.
- Vegetarian-friendly: Choose Gruyère-style cheese and a Parmesan-style hard cheese specifically labeled as made with vegetarian rennet.
Storage & Make-Ahead
Refrigerate: Refrigerate leftovers in a covered container within 2 hours of baking and use within 4 days. Divide a large amount into shallow containers so it cools promptly.
Make ahead: Bake the gratin completely up to 2 days ahead, leaving off the fresh thyme garnish. Refrigerate promptly once the initial heat has dissipated. A metal baking pan is convenient for make-ahead preparation; if using ceramic or glass, follow the manufacturer’s guidance to avoid temperature shock when reheating.
Reheat: Cover the refrigerated gratin with foil and heat at 350°F (175°C) for 35–45 minutes, then uncover for 10 minutes to refresh the crust. The center should reach 165°F (74°C); allow additional covered heating time if needed. Add the fresh thyme just before serving. For individual portions, microwave in a covered, microwave-safe dish in 1-minute intervals until heated through.
Nutrition (per serving)
Estimated for one-eighth of the recipe; actual values vary with the cheeses and dairy used.
Calories: 530 kcal | Carbs: 35g | Protein: 17g | Fat: 35g | Saturated Fat: 22g | Fiber: 4g | Sugar: 5g | Sodium: 630mg | Cholesterol: 110mg
