Quickly knead flour, butter in flakes, 1 egg and 1 teaspoon 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 and cook them in boiling salted water for about 10 minutes.
Onions peel and cut into slices. Mix ricotta, crème fraîche, saffron and 5 eggs. Season with salt and pepper. Roll out the shortcrust pastry on a well floured work surface until round (28 cm Ø).
Grease a tart mould (26 cm Ø) and dust with flour. Line the mould with the short pastry. Spread Brussels sprouts and onions evenly on the pastry. Pour the egg-crème-fraîche over the pastry. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for approx. 45 minutes.
Cover after 30 minutes if necessary. Remove quiche from the oven and garnish with thyme and oregano.