Mix 250 g flour and 1 tsp salt in a bowl. Add 1 egg, 125 g butter and 1-2 tablespoons cold water and knead with the dough hooks of the hand mixer. At the end knead further with your hands to a smooth short pastry.
Roll out the short pastry to a circle (approx. 32 cm Ø) on a floured work surface. Line a greased tart mould with lift-off base (approx. 28 cm Ø) with the pastry, press the edge slightly. Cut off any excess dough.
Prick the bottom of the pastry several times with a fork and chill the mould for 25-30 minutes. Peel onions and cut them into half rings. Grate cheese. Heat 15 g butter in a saucepan. Fry onions until translucent.
Mix 50 g flour and milk in a bowl and pour into the onions. Bring to the boil while stirring and simmer for about 5 minutes. Stir in cheese. Remove the saucepan from the heat. Whisk 2 eggs and sour cream, stir into the hot cheese mixture and season with salt, pepper and nutmeg.
Pour hot cheese mixture on the shortcrust pastry. Cut the camembert into slices and spread on top. Bake in the preheated oven, lower shelf (electric oven: 200 °C/ convection oven: 175 °C/ gas: level 3) for about 40 minutes.
Serve hot or lukewarm. Serve with lettuce.