Equipment
Ingredients
Shortcrust pastry
- 100 g all-purpose flour, plus extra for dusting
- 1 g salt
- 50 g unsalted butter, cold, cut into small pieces
- 30 ml water, cold
Filling
- 50 g unsalted butter
- 750 g yellow onion, peeled and finely sliced
- 9 g fresh thyme, leaves only
- 2 eggs, beaten
- 100 ml heavy cream
- g salt
- g black pepper, freshly ground
Nutrition (per serving)
Nutrition is estimated per serving. Nutrients without complete source data are omitted.
Full nutrition
- Calories
- 313 kcal
- Protein
- 6 g
- Carbohydrates
- 25 g
- Fat
- 21 g
- Fiber
- 3 g
- Sugar
- 6 g
- Sodium
- 230 mg
- Saturated Fat
- 13 g
- Trans Fat
- 0 g
- Monounsaturated Fat
- 6 g
- Polyunsaturated Fat
- 1 g
- Cholesterol
- 117 mg
- Potassium
- 255 mg
- Calcium
- 63 mg
- Iron
- 1.6 mg
- Magnesium
- 22 mg
- Phosphorus
- 103 mg
- Zinc
- 0.6 mg
- Vitamin A
- 213 mcg
- Vitamin C
- 12 mg
- Vitamin D
- 0.6 mcg
- Vitamin E
- 0.8 mg
- Vitamin K
- 3 mcg
- Vitamin B6
- 0.2 mg
- Vitamin B12
- 0.2 mcg
- Thiamin
- 0.21 mg
- Riboflavin
- 0.24 mg
- Niacin
- 1.2 mg
- Folate
- 64 mcg
- Water
- 144 g
Sources: USDA FoodData Central
Method
In a mixing bowl, combine 100g flour and a pinch of salt. Rub in 50g cold butter with your fingertips until the mixture looks like fine breadcrumbs.
Add 15-30ml cold water gradually until a dough forms. Shape into a disc, wrap in plastic, and chill in the refrigerator for 30 minutes.
Preheat the oven to 180°C (350°F).
Melt 50g butter in a frying pan over medium-low heat. Add 750g sliced onions and a pinch of salt.
Cook the onions for 20-30 minutes, stirring frequently, until they are soft and golden brown. Stir in 9g thyme leaves and remove from heat.
Roll out the chilled pastry on a floured surface to the thickness of approximately 3mm.
Line a 20cm or 23cm tart tin with the pastry, pressing it firmly into the corners and trimming any excess.
Bake the empty pastry case at 180°C (350°F) for 10 minutes.
Brush the base and sides of the hot pastry with a small amount of the beaten egg, then bake for another 5 minutes.
In a bowl, whisk the remaining beaten eggs with 100ml heavy cream. Stir in the cooked onions and season with salt and pepper.
Pour the onion and cream mixture evenly into the partially baked pastry shell.
Bake the tart at 180°C (350°F) for 30-35 minutes until the filling is set.
Allow the tart to rest for 10 minutes before slicing and serving.
Chef's Notes
- For the flakiest pastry, ensure all your ingredients, especially the butter and water, are ice-cold. Handle the dough as little as possible to prevent warming the butter.
- Caramelizing onions takes patience. Low and slow is the key to developing deep, sweet flavor without burning. Don't rush this step; it's the heart of the tart.
- Blind baking the tart shell with an egg wash helps to create a barrier, preventing a soggy bottom, especially crucial for a wet filling like this.
- The custard should be just set with a slight wobble in the center when done. Overbaking will result in a rubbery texture.
Storage
Refrigerator: 3 days — Store in an airtight container.
Freezer: 1 month — Wrap tightly in foil and plastic wrap.
Reheating: Reheat in the oven at 150°C (300°F) for 10-15 minutes to maintain pastry crispness.
Troubleshooting
Pastry shrinks down the sides of the tin
Cause: The dough was stretched during lining or not chilled long enough
Fix: Cannot be fixed once baked
Prevention: Avoid pulling the dough when lining the tin and ensure it rests in the fridge for at least 30 minutes
The tart base is soggy
Cause: Onions contained too much moisture or the blind bake was insufficient
Fix: Place the tart on a lower oven rack for the final 10 minutes of baking
Prevention: Cook onions until the liquid has evaporated and ensure the egg wash seal is baked properly
Cultural Context
The Onion and Thyme Tart, or Tarte à l'Oignon, is a classic French dish with roots in the Alsace region. Traditionally a humble peasant dish, it showcases the sweetness of caramelized onions, a staple ingredient in many European cuisines. It's often served as a light meal, appetizer, or as part of a larger spread, especially during cooler months.
