Knead flour, 1 egg, whipped cream, salt, fat in flakes and ground nuts to a smooth dough. Chill for about 30 minutes. Clean and wash the Brussels sprouts and cook in boiling salted water for about 10 minutes.
Peel onions, cut them into slices and fry them in hot oil. Grate cheese coarsely. Drain the Brussels sprouts. Mix remaining eggs, sour cream, mustard and cheese. Season with salt and pepper. Roll out dough between 2 layers of foil to a circle (approx. 31 cm Ø) and press it into a greased tart tin (26 cm Ø).
Add the Brussels sprouts and onions. Pour egg cream over them. Sprinkle with nut leaves. Bake in the preheated oven (electric: 200 °C/ gas: level 3) for about 40 minutes. After about 20 minutes, cover with baking paper if necessary.