1 1/2 cups all purpose flour
pinch of salt
6 tbsp. butter, diced
1 egg yolk

Filling:

4 tbsp. butter
1 lb. onions, thinly sliced
1-2 tbsp. mustard
2 eggs plus 1 egg yolk
1 cup double cream
1/2 cup grated Gruyere cheese
pinch of freshly grated nutmeg
salt and pepper

For the pastry, sift flour and salt into a bowl. Add butter and rub into flour with your fingertips until the mixture resembles fine breadcrumbs. Add egg yolk and 1 tbsp. cold water and mix to a firm dough. Refrigerate for 30 minutes.

Preheat oven to 400°F.

Knead pastry. Roll it out on a floured work surface. Line a 9-inch flan or pie dish. Use a fork to make holes in the pastry, all over and evenly to prevent the crust from puffing up during baking. Line the crust with wax or parchment paper and fill with dry beans (beans can be used over and over again).

Bake pastry for 15 minutes. Remove the paper and beans and bake for another 10 to 15 minutes, or until the crust is crisp.

While the crust is baking, melt butter in a saucepan, add onions, cover with a tight-fitting lid and cook for 20 minutes, stirring occasionally, until golden.

Reduce oven to 350°F. Spread pastry case with mustard and spread onions evenly over crust. Combine eggs, egg yolk, cream cheese, nutmeg and seasoning. Pour over onions.

Bake for 30 to 35 minutes, or until golden.

Serve warm.