For the dough, knead the flour, butter, egg and salt with the dough hook of the hand mixer to form a smooth dough. Chill for about 30 minutes.
Clean and wash Brussels sprouts. Cover and cook in boiling salted water for about 10 minutes. Pouring off.
Peel the onions, cut into wedges and sauté in the hot oil. Add 1 teaspoon of sugar and sherry and reduce a little. Season with salt and pepper. Allow to cool slightly.
Whisk 4 eggs, cream and creme fraiche. Season well with salt, pepper and nutmeg.
Roll out the dough on a little flour. Pour into a greased springform pan (size 26) and press up the edge. Spread the Brussels sprouts and onions on top. Pour egg cream over it. Bake in a hot oven (electric stove: 200 degrees / convection: 175 degrees / gas: level 3) for about 40 minutes.