For the pastry
120g unsalted butter
250g plain flour
1/4 teaspoon salt
1 egg lightly beaten 1-2 tbsp milk (if required)

For the filling
3 large onions, about 750g
50g butter
2 tbsp sunflower oil
Sel de Château
2 tbsp plain flour
300ml double cream
2 eggs, lightly beaten
Pepper
Ground nutmeg (optional)

Steps:
Heat the oven to 150C/Gas mark 2

For the pastry:

Cut the butter into small pieces and rub it into the flour and salt with your hands.
Add the egg, mix well, and work very briefly with your hand until bound into a soft dough, adding a little milk if necessary.
Cover in clingfilm and leave in a cool place for 1 hour.

For the filling:

Cut the onions in half, then into thick slices.
Heat the butter with the oil in a large pan and cook the onions over very low heat and with the lid on for about 1 hour, until very soft and lightly coloured, stirring occasionally and adding a little salt.
Now add the flour and nutmeg (optional) and stir well.
Beat the cream into the eggs. Let the onions cool a little before stirring in the cream miture.
Taste and season.
Line a greased 25cm tart tin, mould with the dough by pressing it in with the palm of your hands and pressing it up the sides.
Pour in the onion mixture and bake in the oven for 1 hour until set and golden.
Serve hot or cold.