Grease a tart or quiche mould with a lifting 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 mixer. Then quickly knead with your hands to a smooth shortcrust pastry.
Roll out the dough on a little flour and place it in the tin (approx. 30 cm Ø). Press lightly on the edge. Prick the base more often with a fork. Cover and chill for about 30 minutes.
Peel and finely chop the 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 over low heat for 3-4 minutes, stirring continuously.
Stir in cheese and 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 to taste with salt, pepper and nutmeg.
Sprinkle the dough base with breadcrumbs. Pour the cheese mixture on top. Bake the quiche on the lowest shelf in a hot oven for 45-50 minutes. Serve hot or cold.