Knead flour, butter, 1 egg and 1 teaspoon of salt with the dough hooks of the hand mixer to a smooth dough. Wrap in foil and chill for 30 minutes. Meanwhile clean and wash the Brussels sprouts, possibly halve them depending on their size and cook in boiling salted water for about 10 minutes. Drain and let drain. Peel onions and cut into slices.
Heat oil in a pan, sauté onions in it and sprinkle with sugar. Deglaze with sherry and boil down for 5-6 minutes. Season with salt and pepper, remove onions and let them cool down. Whisk 4 eggs, cream and crème fraîche, season with salt, pepper and nutmeg. Roll out the shortcrust pastry on a well floured work surface until round (approx. 28 cm Ø). Grease tart tin (26 cm Ø) and dust with flour. Line the tin with the shortcrust pastry. Spread the sprouts and onions on the pastry. Pour egg-cream over the pastry.
Grease tart tin (26 cm Ø) and dust with flour. Line the tin with the shortcrust pastry. Spread the sprouts and onions on the pastry. Pour egg-cream over the pastry. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for about 40 minutes. Take out and serve immediately