Caramelised Onion Sourdough Tart

A golden sourdough pastry crust filled with soft, sweet onions and a custard-like savoury filling — rustic, elegant, and bursting with flavour.

At a Glance

• Loaf Style: Savory tart with sourdough pastry

• Skill Level: Intermediate

• Total Time: ~10–14 hours (includes chill and fermentation)

• Prep Time: ~35 minutes

• Yield: One 9-inch tart (serves 6–8)

• Hydration: N/A

• Bake Temp: 375°F / 190°C

• Baking Vessel: 9-inch tart tin

Ingredients

Sourdough Pastry

• 150g all-purpose flour

• 100g cold unsalted butter, cubed

• 60g sourdough discard (unfed)

• ½ tsp salt

• 1–2 tbsp cold water, as needed

Filling

• 3 large onions (yellow or red), thinly sliced

• 2 tbsp olive oil or butter

• 2 eggs

• 120ml heavy cream

• 60ml milk

• 1 tbsp fresh thyme leaves (or 1 tsp dried)

• Salt and black pepper, to taste

• Optional: 30g grated Gruyère or parmesan

Instructions

Make the Pastry
1. In a bowl, combine flour and salt. Rub in cold butter until mixture resembles coarse crumbs.
2. Add sourdough discard and a tablespoon of water. Mix gently to form a dough — add more water only if needed.
3. Shape into a disc, wrap, and chill for at least 6 hours or overnight.

Prepare the Onions
4. In a skillet, heat olive oil or butter over medium-low heat. Add sliced onions and a pinch of salt.
5. Cook slowly for 25–30 minutes, stirring occasionally, until deeply caramelised and jammy. Set aside to cool.

Roll and Blind Bake the Crust
6. Preheat oven to 375°F / 190°C.
7. Roll the chilled dough into a circle and press into a 9-inch tart tin. Trim excess and prick base with a fork.
8. Line with parchment and fill with baking weights or dry beans. Blind bake for 15 minutes, then remove weights and bake for another 8–10 minutes until lightly golden. Let cool slightly.

Assemble the Filling
9. In a bowl, whisk together eggs, cream, milk, thyme, salt, and pepper. Stir in cooled onions and optional cheese.

Fill and Bake
10. Pour the filling into the pre-baked tart shell. Bake for 25–30 minutes or until the center is just set and golden on top.
11. Cool slightly before slicing.

Visual Cues

• The filling should be lightly golden with visible flecks of onion and herbs.

• The crust should be crisp and deeply golden, with no soggy spots on the base.

Why This Works

Caramelising the onions draws out their natural sweetness and concentrates flavour, which pairs beautifully with the tang of the sourdough crust. The rich custard base balances everything out — it’s comfort food with a refined French twist.