Onion and Gruyère Tart (English)

An onion tart is a classic British pie, the Gruyère blends exceptionally well with the onion and the thyme ties both flavours together. Serve as an appetizer or light lunch, delicious.

Pastry:
butter, cold, cut into cubes (65 g)
all-purpose flour, unbleached (100 g)
1 pinch sea salt
1-2 Tbsp. water, iced

Filling:
butter (50 g)
white onions, peeled and very thinly sliced (750 g)
1 large egg
2 large egg yolks
double cream (200 mL)
2 tsp. Dijon mustard
1 tsp. fresh thyme leaves
sea salt
black pepper, freshly ground
Gruyère, freshly grated (60 g)
freshly grated nutmeg

For the Pastry:
Briefly process the butter, flour and salt together in a food processor until the mixture resembles fine breadcrumbs. Now tip into a large roomy bowl and gently mix in the water with cool hands or table knife until well amalgamated. Knead the dough lightly, then put into a plastic bag and chill in the fridge for at least 1 hour before rolling.

For the Filling:
Melt the butter in a wide, shallow pan. Tip in the onions and very gently sweat over a moderate heat for at least 40 minutes, or as long as 1 hour, until pale golden and completely soft. Cool

Preheat the oven to 350°F (180°C/160°C fan, Gas Mark 4) and place a flat baking sheet inside to heat (it will help the base of the tart to cook thoroughly).

For the Pastry:
Roll out the pastry on a lightly floured surface as thinly as you dare, then use to line a 20 cm tart tin, 3 cm deep. Prick the base with a fork. Line the pastry case with foil and dried beans, slide onto the hot baking sheet and bake 'blind' for about 15-20 minutes. Remove the foil and beans and return the pastry case to the oven for a further 10 minutes or so, until it is golden, crisp and well cooked through, particularly the base.

Assembly:
Mix together the egg, egg yolks, cream, mustard and thyme leaves. Carefully mix in the onions, season and pile the mixture into the pastry case. Sprinkle the Gruyère over the surface and generously grate over some nutmeg.

Bake in the preheated oven for about 45 minutes, or until the filling is a rich golden colour and just firm to the touch. Leave to stand for 10 minutes before eating.

Makes 4-6 servings.