Tarte à l'Oignon (French Onion Tart)

A golden tart of slowly softened onions in a creamy egg custard with thyme, baked in crisp shortcrust pastry.

TIME BREAKDOWN
Prep: 30 min
Cook: 1h 15m
Total: 1 hour 45 minutes
Vegetarian
✅
Nuts
❌
Serves
6 people
Difficulty
Easy

Ingredients

  • For the pastry: 200g plain flour, plus extra for dusting
  • For the pastry: 100g cold unsalted butter, cubed
  • For the pastry: 1 egg yolk
  • For the pastry: 2 to 3 tbsp ice-cold water
  • For the pastry: pinch of salt
  • 30g unsalted butter
  • 1 tbsp olive oil
  • 800g onions, thinly sliced
  • 2 sprigs fresh thyme, leaves picked
  • 3 large eggs
  • 200ml double cream
  • 100ml whole milk
  • 60g Gruyère, grated
  • Pinch of freshly grated nutmeg
  • Salt and black pepper to taste

Method

  1. Rub the flour, butter and salt together until the mixture resembles breadcrumbs. Mix in the egg yolk and enough water to make a dough, then wrap and chill for 30 minutes.

  2. Heat the oven to 200°C (180°C fan). Roll the pastry out and line a 23cm loose-bottomed tart tin, 4cm deep. Prick the base, line with baking paper and baking beans, and bake blind for 15 minutes. Remove the paper and beans and bake for 5 more minutes. Lower the oven to 180°C (160°C fan).

  3. Melt the butter with the oil in a large pan over a low heat. Add the onions, thyme and a pinch of salt, cover and cook for 15 minutes, then uncover and cook for 20 minutes more, stirring often, until golden and very soft. Cool for 10 minutes.

  4. Whisk the eggs, cream, milk and nutmeg with salt and pepper. Spread the onions over the pastry case and sprinkle with the cheese, then pour in the custard.

  5. Bake for 30 to 35 minutes until golden and just set. Rest for 10 minutes and serve warm or at room temperature with a green salad.