Quick Recipe Version (TL;DR)
Quick Ingredients
- All-purpose flour: 150 g (1 1/4 cups)
- Kosher salt: 1 tsp, divided
- Unsalted butter (cold): 113 g (8 tbsp)
- Egg yolk: 1 large
- Ice water: 30–45 ml (2–3 tbsp)
- Yellow onions: 680 g (about 3 large), thinly sliced
- Olive oil: 1 tbsp
- Unsalted butter: 2 tbsp
- Sugar (optional): 1/4 tsp
- Balsamic vinegar: 2 tsp
- Fresh thyme leaves (or dried): 1 tsp fresh (or 1/2 tsp dried)
- Eggs: 4 large
- Heavy cream: 240 ml (1 cup)
- Whole milk: 120 ml (1/2 cup)
- Black pepper: 1/4 tsp
- Ground nutmeg (optional): 1/8 tsp
- Gruyère cheese, shredded: 150 g (about 1 1/2 cups)
Do This
- 1) Make tart dough, chill 20 minutes.
- 2) Caramelize onions low and slow (about 45 minutes); finish with balsamic and thyme.
- 3) Roll dough, fit into a 9-inch tart pan, chill 10 minutes.
- 4) Blind-bake crust at 190°C (375°F) for 20 minutes; remove weights and bake 5 minutes more.
- 5) Whisk eggs, cream, milk, salt, pepper (and nutmeg); stir in cheese.
- 6) Add onions to crust, pour custard over; bake at 180°C (350°F) for 30–35 minutes until just set.
- 7) Cool 15 minutes before slicing for clean wedges.
Why You’ll Love This Recipe
- The onions cook down into a deep, sweet-savory layer that tastes far more fancy than the effort involved.
- A crisp, buttery crust stays sturdy under the creamy egg filling.
- Perfect for brunch, lunch with salad, or an easy vegetarian dinner.
- It reheats beautifully, so leftovers are genuinely something to look forward to.
Grocery List
- Produce: 3 large yellow onions, fresh thyme (optional)
- Dairy: unsalted butter, heavy cream, whole milk, Gruyère cheese
- Pantry: all-purpose flour, kosher salt, olive oil, sugar (optional), balsamic vinegar, black pepper, ground nutmeg (optional)
Full Ingredients
For the Crisp Tart Crust (9-inch / 23 cm)
- All-purpose flour: 150 g (1 1/4 cups), spooned and leveled
- Kosher salt: 1/2 tsp
- Unsalted butter: 113 g (8 tbsp), very cold, cut into 1/2-inch cubes
- Egg yolk: 1 large
- Ice water: 30–45 ml (2–3 tbsp), as needed
For the Caramelized Onions
- Yellow onions: 680 g (about 3 large), halved and thinly sliced
- Unsalted butter: 2 tbsp
- Olive oil: 1 tbsp
- Kosher salt: 1/2 tsp
- Sugar (optional): 1/4 tsp (helps browning, especially if your onions are mild)
- Balsamic vinegar: 2 tsp
- Fresh thyme leaves: 1 tsp (or 1/2 tsp dried thyme)
For the Egg Custard Filling
- Large eggs: 4
- Heavy cream: 240 ml (1 cup)
- Whole milk: 120 ml (1/2 cup)
- Kosher salt: 1/2 tsp
- Black pepper: 1/4 tsp
- Ground nutmeg (optional): 1/8 tsp
- Gruyère cheese, shredded: 150 g (about 1 1/2 cups), divided
Step-by-Step Instructions
Step 1: Make the tart dough
In a large bowl, whisk together the flour and 1/2 tsp kosher salt. Add the cold butter cubes and cut them in using a pastry cutter or your fingertips until the mixture looks like coarse crumbs with some pea-size pieces (those butter pieces create flakiness).
Add the egg yolk and drizzle in 2 tbsp (30 ml) ice water. Mix with a fork until the dough begins to clump. If it still looks dry, add the remaining 1 tbsp (15 ml) ice water, a teaspoon at a time, just until it holds together when you press it.
Form into a flat disk, wrap tightly, and refrigerate for 20 minutes.
Step 2: Caramelize the onions low and slow
While the dough chills, set a large skillet over medium-low heat. Add the butter and olive oil. When the butter melts, add the sliced onions, 1/2 tsp kosher salt, and the optional sugar.
Cook for 45 minutes, stirring every 3–5 minutes. You’re looking for onions that are deeply golden-brown and jammy, not just softened. If the pan starts to look dry or the onions are browning too quickly in spots, reduce the heat slightly and stir more often.
Stir in the balsamic vinegar and thyme, scraping up any browned bits, and cook for 1 minute. Remove from the heat and let cool slightly while you prep the crust.
Step 3: Roll the crust and fit the pan
Lightly flour a clean work surface. Roll the dough into a 30 cm (12-inch) circle, about 3 mm (1/8-inch) thick. Carefully lift it into a 9-inch (23 cm) tart pan with a removable bottom. Press it into the corners without stretching the dough (stretching can cause shrinkage).
Trim the excess by rolling your pin over the top edge or trimming with a knife. Chill the shaped crust in the refrigerator for 10 minutes while you preheat the oven.
Step 4: Blind-bake for a crisp crust
Preheat the oven to 190°C (375°F).
Line the chilled tart shell with parchment paper and fill with pie weights or dried beans/rice (enough to fully cover the base and go slightly up the sides).
Bake for 20 minutes. Carefully lift out the parchment and weights, then bake for 5 minutes more to dry and crisp the base.
Reduce oven temperature to 180°C (350°F).
Step 5: Whisk the egg custard
In a medium bowl, whisk the eggs until well combined. Whisk in the heavy cream, milk, 1/2 tsp kosher salt, black pepper, and optional nutmeg.
Stir in about 3/4 of the shredded Gruyère (save the rest for the top).
Step 6: Assemble the tart
Spread the caramelized onions evenly over the warm blind-baked crust. Sprinkle the remaining Gruyère over the onions (this helps create a flavorful layer and keeps the custard from soaking the crust).
Slowly pour the egg mixture over the onions. If your tart pan is very full, place it on the oven rack first, then pour to avoid spills.
Step 7: Bake until just set, then cool for clean slices
Bake at 180°C (350°F) for 30–35 minutes, or until the center is just set. The tart should look mostly firm with a slight wobble in the very center (it will finish setting as it cools). If the crust edges are browning too quickly, cover them loosely with foil for the last 10 minutes.
Cool on a rack for 15 minutes before removing the tart ring and slicing. Serve warm or at room temperature.
Pro Tips
- Don’t rush the onions: Keep the heat medium-low and give them the full 45 minutes. Fast heat gives you sautéed onions, not caramelized onions.
- Blind-baking matters: Those extra 5 minutes without weights dry the base so it stays crisp under the custard.
- Custard texture: Bake until there’s only a small jiggle in the center. Overbaking can make the filling rubbery.
- Grate your own cheese: Pre-shredded cheese often has anti-caking agents that melt less smoothly in custards.
- Easy transfer: Bake the tart pan on a sheet pan for stability and simpler in-and-out of the oven.
Variations
- Herb swap: Replace thyme with 1 tbsp chopped chives or 1 tsp chopped rosemary (rosemary is strong; use less).
- Cheese options: Try 150 g (1 1/2 cups) sharp cheddar for a bolder taste, or goat cheese (crumble 85 g / 3 oz over the onions and reduce Gruyère to 85 g / 3 oz).
- Add greens: Wilt 90 g (3 packed cups) baby spinach, squeeze dry, then scatter over the onions before adding custard.
Storage & Make-Ahead
Refrigerate: Cool completely, then cover and refrigerate for up to 4 days. Reheat slices on a sheet pan at 160°C (325°F) for 10–12 minutes, or until warmed through.
Make ahead: Caramelize the onions up to 3 days ahead and refrigerate. You can also blind-bake the crust up to 1 day ahead; cool completely, cover tightly at room temperature, then assemble and bake the next day.
Freeze: Wrap baked, cooled slices tightly and freeze up to 2 months. Thaw overnight in the fridge and reheat at 160°C (325°F) for 12–15 minutes.
Nutrition (per serving)
Approximate per slice (1/8 tart): 380 calories, 12 g protein, 22 g carbohydrates, 28 g fat, 2 g fiber, 4 g sugar, 430 mg sodium.
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