Peel and finely dice the onion and garlic. Heat 2 tablespoons of oil. Sauté onion and garlic in it, add frozen spinach and 4 tablespoons of water. Defrost the spinach at low heat covered.
In the meantime, whisk the sour cream and eggs, preheat the oven (electric cooker: 200 °C/circulating air: 180 °C/gas: see manufacturer). Spread a tart tin (24 cm Ø) with a little oil. Roll out the dough sheets and place them offset in the form so that the form is laid out with the rim. Spread each sheet with approx. 1 tbsp. oil.
Put the spinach in a sieve and squeeze it out really well with a spoon. Stir the spinach into the egg cream. Season with salt, pepper and nutmeg. Spread the mixture in the mould. Crumble feta over it. Bake in a hot oven for about 45 minutes.