For the short pastry, mix 250 g flour and 1 teaspoon salt. Add 1 egg, 125 g butter in pieces and 1-2 tbsp. ice-cold water. Knead first with the dough hooks of the mixer, then with your hands briefly to a smooth dough.
Cover the dough and chill for about 30 minutes.
In the meantime peel and finely dice the onions. Heat 1 tablespoon butter in a pot and fry the onions in it. Sprinkle with 50 g flour and sauté briefly. Deglaze with milk and bring to the boil. Simmer for about 5 minutes while stirring.
Grate the Comté and Raclette and melt in the hot sauce while stirring. Pull the pot off the stove. Whisk 2 eggs and the sour cream. Stir into the cheese mixture (no longer boiling!). Season to taste with salt, pepper and nutmeg.
Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Grease a tart mould with lifting base (approx. 28 cm Ø). Roll out the dough to a round shape on some flour (approx. 32 cm Ø). Line the tart tin with the dough, pull up the edge and press down lightly.
Cut the edge straight. Prick the pastry base several times with a fork and sprinkle with breadcrumbs.
Pour cheese sauce on the dough. Cut the camembert into thin slices and spread on top. Bake in a hot oven on the lowest shelf for 35-40 minutes. Serve hot or lukewarm.