For the short pastry, mix 250 g flour and 1 teaspoon salt. Add 1 egg, 125 g butter in pieces and 1-2 tbsp. cold water. Knead first with the dough hooks of the mixer, then with your hands to a smooth dough.
Cover and chill for about 30 minutes.
In the meantime peel and finely dice the onion. Heat 1 tablespoon butter in a pot. Sauté the onion in it. Add frozen spinach and 150 ml water, bring to the boil, cover and stew for about 15 minutes. Stir from time to time.
Then continue to simmer open for about 5 minutes until all the liquid has evaporated. Season to taste with salt, pepper and nutmeg.
Preheat oven (electric cooker: 200°C/circulating air: 175°C/gas: level 3). Grease a tart mould with lifting base (approx. 28 cm Ø). Roll out the dough on a little flour (approx. 32 cm Ø). Line the tart mould with the dough, pull up the edge and press down lightly.
Prick the pastry base several times with a fork and sprinkle with breadcrumbs.
Wash the tomatoes. Spread the spinach and tomatoes on the dough. Whisk milk, sour cream and 4 eggs. Grate cheese and stir in. Season with salt, pepper and nutmeg. Pour egg cream over the vegetables. Cut mozzarella into slices and cover the quiche with it.
Bake in the hot oven on the middle shelf for 40-50 minutes. Sprinkle with pine nuts about 5 minutes before the end of the baking time.