Quick Recipe Version (TL;DR)
Quick Ingredients
- 1 unbaked 9-inch pie crust (homemade or store-bought)
- 3 large yellow onions (about 2 lb / 900 g), thinly sliced
- 2 tbsp (28 g) unsalted butter
- 1 tbsp (15 ml) olive oil
- 1/2 tsp kosher salt (for onions), plus 1/2 tsp (for custard)
- 1/2 tsp granulated sugar (optional, helps browning)
- 1/4 cup (60 ml) dry white wine or water (to deglaze)
- 2 tsp fresh thyme leaves (or 1 tsp dried thyme)
- 3 large eggs
- 1 cup (240 ml) heavy cream
- 1/2 cup (120 ml) whole milk
- 1/4 tsp ground nutmeg
- 1/2 tsp black pepper
- 6 oz (170 g) Gruyère, grated (about 1 1/2 cups)
- 1 tsp Dijon mustard (optional, for the crust)
Do This
- 1. Blind-bake crust at 375°F (190°C): 15 min with weights, 10 min without; cool 10 min.
- 2. Caramelize onions in butter + oil with salt (and optional sugar) over low heat, 45–55 min; deglaze with wine/water; cool 10 min.
- 3. Whisk eggs, cream, milk, salt, pepper, nutmeg, and thyme.
- 4. Brush crust with Dijon (optional). Add half the Gruyère, then onions, then remaining Gruyère.
- 5. Pour custard over filling. Bake at 350°F (175°C) for 35–40 min until gently set.
- 6. Rest 15–20 min before slicing for the cleanest cuts and best texture.
Why You’ll Love This Recipe
- Deeply savory-sweet flavor from properly slow-caramelized onions.
- Custardy (not watery) filling that sets delicately and slices neatly.
- Gruyère melts into a nutty, golden top that tastes like a bistro tart.
- Great warm, room temp, or reheated—perfect for brunch, lunch, or dinner.
Grocery List
- Produce: 3 large yellow onions, fresh thyme (optional but recommended)
- Dairy: unsalted butter, Gruyère cheese, heavy cream, whole milk, eggs
- Pantry: pie crust (or flour/salt/sugar for homemade), olive oil, kosher salt, black pepper, ground nutmeg, Dijon mustard (optional), dry white wine (or water)
Full Ingredients
Flaky Pie Shell (Homemade Option) (Makes 1 single 9-inch crust)
- 1 1/4 cups (150 g) all-purpose flour
- 1/2 tsp kosher salt
- 1 tsp granulated sugar (optional; helps browning)
- 1/2 cup (113 g) unsalted butter, cold and cut into 1/2-inch cubes
- 3 to 5 tbsp (45 to 75 ml) ice water
Caramelized Onion Filling
- 3 large yellow onions (about 2 lb / 900 g), halved and thinly sliced
- 2 tbsp (28 g) unsalted butter
- 1 tbsp (15 ml) olive oil
- 1/2 tsp kosher salt
- 1/2 tsp granulated sugar (optional; speeds browning a little)
- 1/4 cup (60 ml) dry white wine (or water or unsalted broth)
- 2 tsp fresh thyme leaves (or 1 tsp dried thyme)
Custard + Assembly
- 6 oz (170 g) Gruyère cheese, grated (about 1 1/2 cups), divided
- 3 large eggs
- 1 cup (240 ml) heavy cream
- 1/2 cup (120 ml) whole milk
- 1/2 tsp kosher salt
- 1/2 tsp freshly ground black pepper
- 1/4 tsp ground nutmeg
- 1 tsp Dijon mustard (optional; brushed on crust for extra savoriness)
Optional Finish
- 1 tsp fresh thyme leaves, for sprinkling after baking
Step-by-Step Instructions
Step 1: Make (or prep) the crust
If using store-bought crust, place it in a 9-inch pie plate (or a 9-inch tart pan with removable bottom), crimp/shape the edge, then chill in the refrigerator for 30 minutes while you start the onions.
If making homemade crust: In a medium bowl, whisk together the flour, salt, and optional sugar. Cut in the cold butter with a pastry cutter (or your fingertips) until the mixture looks like coarse crumbs with a few pea-size butter pieces. Drizzle in 3 tbsp (45 ml) ice water and gently mix. Add more ice water, 1 tbsp at a time, just until the dough holds together when squeezed (usually 4–5 tbsp total).
Form into a 1-inch-thick disk, wrap, and chill for 30 minutes. Roll to a 12-inch circle (about 1/8-inch thick), fit into your pan, trim, and crimp. Chill the shaped crust for 30 minutes.
Step 2: Caramelize the onions slowly
Set a large skillet (12-inch is ideal) over medium heat. Add the butter and olive oil. Once the butter melts, add the sliced onions and sprinkle with 1/2 tsp kosher salt (and the optional 1/2 tsp sugar). Toss to coat.
Reduce heat to low and cook, stirring every 3–5 minutes at first and more often toward the end, until the onions are deeply golden and jammy, 45–55 minutes. If onions start to brown too quickly or stick, lower the heat and add 1–2 tbsp water as needed.
Step 3: Deglaze and season the onions
Once the onions are richly caramelized, add the 1/4 cup (60 ml) wine (or water) and scrape up any browned bits from the pan. Cook 1–2 minutes until the liquid is mostly evaporated.
Stir in the thyme. Taste and adjust lightly if needed (remember the cheese adds salt). Spread onions on a plate or shallow bowl and let cool for 10 minutes so they don’t overheat the custard.
Step 4: Blind-bake the crust
Place a rack in the middle of the oven. Preheat to 375°F (190°C).
Line the chilled crust with parchment paper and fill with pie weights or dried beans. Bake for 15 minutes. Carefully lift out the parchment and weights, then bake 10 minutes more, until the bottom looks dry and just barely turning pale golden.
Let the crust cool for 10 minutes. Reduce oven temperature to 350°F (175°C).
Step 5: Make the custard base
In a large bowl (or a 4-cup liquid measuring pitcher), whisk the eggs until smooth. Whisk in the heavy cream, milk, 1/2 tsp kosher salt, black pepper, nutmeg, and any remaining thyme (if some stuck to the onions, that’s fine). Whisk until fully combined.
Step 6: Assemble for even layers and clean slices
If using, brush the inside of the par-baked crust with 1 tsp Dijon mustard (a thin, even layer). Sprinkle half of the Gruyère (about 3 oz / 85 g) over the bottom—this helps create a barrier and keeps the crust flaky.
Spoon the cooled caramelized onions over the cheese and spread into an even layer. Sprinkle the remaining Gruyère over the onions.
Place the pie/tart pan on a rimmed baking sheet (for easy transport and to catch drips). Slowly pour the custard over the filling. Pause for a few seconds so it settles into the onions; add the last bit of custard as space allows.
Step 7: Bake until delicately set and deeply golden
Bake at 350°F (175°C) for 35–40 minutes, rotating the pan at the 20-minute mark. The tart is done when the edges are set, the center has only a slight wobble (like set gelatin), and the top is deeply golden in spots.
If the crust edge is browning too quickly, loosely cover the rim with foil for the last 10–15 minutes.
Step 8: Rest, slice, and serve
Let the tart rest for 15–20 minutes before slicing. This resting time finishes the set and gives you cleaner slices.
Serve warm or at room temperature. If you like, sprinkle with 1 tsp fresh thyme leaves just before serving.
Pro Tips
- Go low and slow on the onions: True caramelization takes 45–55 minutes. If you rush them, you’ll get sautéed onions (still tasty, but not the same savory-sweet depth).
- Cool the onions before assembling: Hot onions can start cooking the eggs early and lead to a slightly grainy custard.
- Par-bake fully enough to dry the base: That second blind-bake (without weights) helps prevent a soggy bottom.
- Use freshly grated Gruyère: Pre-shredded cheese often contains anti-caking agents that melt less smoothly.
- Know the “set” cue: Pull it when the center still has a gentle wobble; carryover heat finishes the custard without overbaking.
Variations
- Add bacon or ham: Sprinkle 4 oz (115 g) cooked, crumbled bacon or diced ham over the onions before adding custard.
- Swap the cheese: Replace half the Gruyère with sharp white cheddar or Comté for a slightly different nutty profile.
- Make it more herby: Add 1 tbsp chopped chives or 1 tsp chopped fresh rosemary (use rosemary sparingly; it’s strong).
Storage & Make-Ahead
Refrigerate: Cool completely, then cover and refrigerate for up to 4 days.
Reheat: Warm slices on a baking sheet in a 325°F (165°C) oven for 12–15 minutes, or until heated through. (Microwaving works in a pinch but softens the crust.)
Make-ahead options: Caramelize the onions up to 3 days ahead and refrigerate. You can also blind-bake the crust 1 day ahead; store at room temperature, loosely covered. Assemble and bake the day you want to serve for the best texture.
Nutrition (per serving)
Approximate (1 of 8 slices): 420 calories; 31 g fat; 24 g carbohydrates; 12 g protein; 2 g fiber; 520 mg sodium. Values vary by crust brand and exact cheese used.
