Grease a tart or quiche dish with a removable base (approx. 26 cm Ø) and dust with flour. Mix 250 g flour and about 1 teaspoon salt in a bowl. Add 1 egg, 125 g butter and 1-2 tbsp. ice-cold water.
First work through everything with the dough hooks of the hand mixer. Then quickly knead with your hands to a smooth shortcrust pastry.
Roll out the dough on a little flour and put it into the tin (approx. 30 cm Ø). Press lightly on the edge. Prick the base several times with a fork. Cover with foil and chill for about 30 minutes.
##peel and finely dice onion##. Grate both ##cheeses##. Heat 2 tablespoons of butter in a saucepan. Sauté onion in it. Stir 50 g flour and milk until smooth, pour into the onion.
Bring to the boil while stirring, simmer for 3-4 minutes at low heat while stirring continuously. Stir in cheese, melt.
Preheat oven (electric cooker: 175°C/ circulating air: 150°C/ gas: level 2). Mix sour cream and 3 eggs. Stir into the hot cheese mixture (do not boil any more!). Season with salt, pepper and ##muscat##.
Sprinkle the dough base with breadcrumbs. Pour cheese mixture on top. Bake the quiche on the lowest shelf in a hot oven for 45-50 minutes. Serve hot or cold.