Quick Recipe Version (TL;DR)
Quick Ingredients
- 2 lb (900 g) Yukon Gold potatoes, peeled and sliced 1/8 in (3 mm)
- 2 medium leeks (white/light green only), thinly sliced
- 2 cups (8 oz/225 g) grated Gruyère
- 1/4 cup (1 oz/30 g) finely grated Parmesan (optional)
- 1 1/2 cups (360 ml) heavy cream + 1 cup (240 ml) whole milk
- 3 tbsp unsalted butter
- 3 garlic cloves (1 halved, 2 minced)
- 1 tsp fresh thyme leaves (or 1/2 tsp dried)
- 1/4 tsp freshly grated nutmeg
- 1 1/4 tsp kosher salt, 1/2 tsp black pepper
Do This
- 1. Heat oven to 375°F (190°C). Butter a 9×13 in (3-qt) baking dish; rub with halved garlic.
- 2. Sauté leeks in 2 tbsp butter until tender, 6–8 min; add minced garlic for 30 sec. Season with a pinch of salt.
- 3. Warm cream, milk, nutmeg, thyme, 1 tsp salt, and pepper to a bare simmer; remove from heat.
- 4. Peel and slice potatoes 1/8 in thick; rinse and pat very dry.
- 5. Layer 1/3 potatoes, 1/2 leeks, 1/3 Gruyère; repeat. Top with remaining potatoes and Gruyère.
- 6. Pour warm cream mixture evenly over layers. Dot with 1 tbsp butter. Cover tightly with foil.
- 7. Bake 45 min covered; uncover, sprinkle Parmesan, bake 20–25 min more. Broil 1–3 min until deep golden. Rest 15 min before serving.
Why You’ll Love This Recipe
- Classic holiday comfort: tender potatoes, sweet leeks, and nutty Gruyère in a silky, fragrant cream.
- Balanced richness: a whisper of nutmeg and thyme keeps each bite bright, not heavy.
- Reliable technique: warmed dairy and thin slices ensure even cooking and a lush, set gratin.
- Showstopping finish: a quick broil creates a bubbling, golden crust worthy of your Christmas table.
Grocery List
- Produce: Yukon Gold potatoes, leeks, garlic, fresh thyme (or dried), chives for garnish (optional)
- Dairy: Gruyère, Parmesan (optional), heavy cream, whole milk, unsalted butter
- Pantry: Kosher salt, black pepper, whole nutmeg (or ground)
Full Ingredients
For the gratin
- 2 lb (900 g) Yukon Gold potatoes, peeled and sliced 1/8 in (3 mm) thick
- 2 medium leeks, white and light green parts only, thinly sliced and thoroughly rinsed (about 2 cups/200 g)
- 2 cups (8 oz/225 g) Gruyère, coarsely grated
- 1 1/2 cups (360 ml) heavy cream
- 1 cup (240 ml) whole milk
- 3 tbsp unsalted butter, divided
- 3 garlic cloves: 1 clove halved (for rubbing the dish) + 2 cloves minced
- 1 tsp fresh thyme leaves (or 1/2 tsp dried)
- 1/4 tsp freshly grated nutmeg
- 1 1/4 tsp kosher salt, divided
- 1/2 tsp freshly ground black pepper
For finishing
- 1/4 cup (1 oz/30 g) finely grated Parmesan (optional, for an extra-savory top)
- Finely snipped chives or extra thyme leaves, for garnish (optional)
Step-by-Step Instructions
Step 1: Preheat and prepare the baking dish
Heat the oven to 375°F (190°C) with a rack in the center. Lightly butter a 9×13-inch (3-quart) baking dish with 1 teaspoon of the butter. Rub the entire inside of the dish with the cut side of the halved garlic clove for a subtle garlicky base.
Step 2: Sauté the leeks
In a large skillet over medium heat, melt 2 tablespoons butter. Add the sliced leeks and a pinch of salt. Cook, stirring occasionally, until very tender and sweet but not browned, 6–8 minutes. Stir in the minced garlic and cook 30 seconds until fragrant. Remove from heat.
Step 3: Warm the cream mixture
In a small saucepan, combine heavy cream, milk, thyme, nutmeg, 1 teaspoon kosher salt, and the black pepper. Warm over medium heat just to a bare simmer, 3–4 minutes; do not boil. Turn off the heat and let stand while you prepare the potatoes. This step perfumes the dairy and helps prevent curdling.
Step 4: Slice and dry the potatoes
Peel the potatoes and slice 1/8 inch (3 mm) thick using a mandoline for precision or a sharp knife. Rinse the slices briefly under cold water to remove excess starch, then pat very dry with clean towels—dry slices layer better and help the cream thicken nicely.
Step 5: Build the layers
Arrange one-third of the potato slices in the dish, slightly overlapping like shingles. Top with half the sautéed leeks and one-third of the Gruyère. Repeat with another one-third of potatoes, the remaining leeks, and another third of Gruyère. Finish with the remaining potatoes and the last third of Gruyère.
Slowly pour the warm cream mixture evenly over the layers. Dot the top with the remaining 1 teaspoon butter (or small pieces from what’s left of the 3 tbsp). Gently press down to ensure the cream reaches all layers.
Step 6: Bake covered
Cover the dish tightly with foil (or foil lined with parchment to prevent sticking). Bake for 45 minutes. The potatoes should be starting to soften and the edges will look creamy and bubbly.
Step 7: Uncover, brown, and broil
Remove the foil. Sprinkle the Parmesan evenly over the top (if using). Return to the oven and bake uncovered until the potatoes are completely tender and the top is richly golden, 20–25 minutes more. For a restaurant-style finish, switch to broil on high and broil 1–3 minutes until the cheese is deeply blistered and browned—watch closely.
Step 8: Rest and serve
Let the gratin rest 15 minutes so the creamy sauce sets into silky layers. Garnish with snipped chives or extra thyme. Serve warm, making sure each scoop captures the golden, bubbling top and tender, fragrant layers beneath.
Pro Tips
- Uniform slices are key. A mandoline set to 1/8 inch (3 mm) ensures even cooking and neat layers.
- Warm the dairy first. Starting with warm cream and milk prevents curdling and speeds the bake.
- Dry the potatoes thoroughly after rinsing. Excess water will thin the sauce.
- Season in the dairy, not on each layer. Evenly seasoned cream distributes flavor without oversalting.
- Foil-first, broil-last. Covering cooks the potatoes; a quick broil creates that irresistible bubbling, golden crust.
Variations
- Smoky bacon-leek: Crisp 4 oz (115 g) diced bacon or pancetta; cook leeks in the drippings plus 1 tbsp butter. Reduce added salt slightly.
- Mushroom-thyme: Sauté 8 oz (225 g) sliced cremini with the leeks until browned; adds savory depth and a woodsy aroma.
- Cheese swap: Try Comté or Emmental for a classic Alpine note, or a blend of Gruyère and sharp white cheddar for extra tang.
Storage & Make-Ahead
Assemble up to 1 day ahead: build the gratin, pour over the warm cream, and cool completely. Cover tightly and refrigerate. Bake an additional 10–15 minutes if starting cold. Leftovers keep 3–4 days refrigerated; reheat, covered, at 350°F (175°C) for 20–25 minutes until hot, then uncover to re-crisp for 5 minutes. Freezing is possible but may slightly change the texture of the potatoes and cream—reheat from thawed at 350°F until bubbling.
Nutrition (per serving)
Approx. 430 calories; 31 g fat; 27 g carbohydrates; 14 g protein; 2 g fiber; 780 mg sodium. Values are estimates.
