Finely grate the cheese. For the dough, knead 350 g flour, 100 g cheese, 200 g butter in pieces, crème fraîche, lemon juice and 1 teaspoon salt in a bowl with the dough hooks of the mixer. On some flour knead with your hands to a smooth dough. Wrap in foil and chill for about 1 hour.
In the meantime peel the onions. Clean or peel zucchini, carrots and potatoes. Dice everything. Heat the oil in a large pan. Fry the potatoes for about 10 minutes, turning them over. Add onions, carrots and courgettes and fry for 5-8 minutes. Season with salt and pepper. Drain on kitchen paper.
For the sauce, mix 50 g flour and milk in a saucepan until smooth. Bring to the boil while stirring and simmer at low heat for about 5 minutes. Melt the rest of the cheese in it. Remove the pot from the stove. Whisk eggs and sour cream and stir into the sauce. Season with salt, pepper and nutmeg.
Roll out the dough on a little flour and make it round (about 40 cm Ø). Place in a greased springform pan (28 cm Ø; approx. 7 cm high) and press down. Prick the base several times with a fork and sprinkle with breadcrumbs.
Spread the vegetable mixture on the dough and pour the cheese sauce over it. Bake in the preheated oven (electric oven: 200 °C/circulating air: 175 °C/gas: see manufacturer) on the bottom rack for 40-45 minutes. Cover with aluminium foil after about half of the baking time. Allow to rest for 10-15 minutes, then remove from the mould. Serve with crème fraîche.