French Onion Tart (Tarte à l'Oignon) Recipe
Silky caramelised onion custard in a buttery shortcrust pastry shell — a classic Alsatian French onion tart. This easy French onion tart is perfect for a sophisticated starter, lunch or light dinner.
About This Recipe
The French onion tart from Alsace is one of those dishes where patience transforms something humble (an onion) into something extraordinary. The onions must be caramelised over very low heat for 40–50 minutes until golden, sweet and jammy. Combined with cream and eggs, they become a silky, intensely flavoured custard in a crisp, buttery pastry shell.
Ingredients
Serves 6
- 200 gplain flour(for pastry)
- 100 gcold unsalted butter(cubed, for pastry)
- 1 egg yolkegg yolk(for pastry)
- 2 tbspcold water
- 1 kgwhite onions(thinly sliced (about 5 large))
- 50 gunsalted butter(for caramelising)
- 1 tspcaster sugar
- 3eggs
- 200 mldouble cream
- 0.5 tspground nutmeg
- 1 tspsalt
Instructions
- 1
Make the pastry
Rub butter into flour until fine crumbs. Mix in egg yolk and cold water. Form into a disc, wrap and refrigerate 30 minutes. Roll and line a 23cm tart tin. Blind bake at 190°C for 15 minutes with baking beans, then 5 more minutes uncovered.
- 2
Caramelise the onions
Melt butter in a large pan over very low heat. Add onions, salt and sugar. Cook, stirring every 5 minutes, for 45–50 minutes until deep golden, very soft and jammy. This cannot be rushed.
The Maillard reaction that creates the caramelised flavour needs time at low heat. High heat burns the edges before the inside softens.
- 3
Make the custard and bake
Whisk eggs, cream, nutmeg and salt. Stir in caramelised onions. Pour into the blind-baked shell. Bake at 180°C for 30–35 minutes until set with a slight wobble. Cool 10 minutes before slicing.
Pro Tips
- →
The onions will reduce dramatically — 1kg raw becomes about 300g caramelised. Don't be alarmed.
- →
Don't rush caramelising. Onions cooked fast become sweet but lack the complex, jammy flavour.
- →
A slight wobble in the centre when you take the tart out is correct — it sets as it cools.
Variations
- •
With Gruyère: scatter 60g grated Gruyère over the onions before pouring the custard.
- •
With thyme: add 1 tsp fresh thyme leaves to the caramelised onions.
Storage
Keep refrigerated for 3 days. Serve at room temperature or warm in a 160°C oven for 10 minutes.
History & Origin
Onion tart is a specialty of Alsace, the region on the French-German border, where similar dishes appear in German (Zwiebelkuchen) and French versions. The abundance of onions in the region and the local cream made it a natural pairing. It remains a fixture of Alsatian restaurants and home cooking.
Frequently Asked Questions
How do I know when onions are properly caramelised?
They should be deep golden brown, very soft, reduced to about a quarter of their original volume, and taste sweet rather than pungent. This takes 40–50 minutes — don't rush it.
Can I use a shop-bought pastry case?
Yes — a ready-made shortcrust pastry case saves significant time and works well.
What is the difference between French onion tart and quiche?
A quiche always has a cream and egg custard base but can have many fillings. The French onion tart specifically features caramelised onions as the primary ingredient and the custard is secondary.
Can the tart be made ahead?
Yes — bake the day before, refrigerate, and bring to room temperature or warm before serving.
Nutrition Facts
Per serving (350g) · 6 servings total
Time Summary
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