Defrost the spinach. Mash the ricotta, crème fraiche and milk. Season with salt, pepper and nutmeg. Peel and finely dice onions and garlic. Clean, wash and chop the mushrooms.
Heat 1 tablespoon of oil in a large frying pan. Fry the minced meat until crumbly. Fry the mushrooms, half onions and half garlic. Stir in strained tomatoes and oregano, bring to the boil. Season with salt and pepper.
Heat 1 tablespoon of oil in a saucepan. Sauté the remaining onions and garlic until transparent. Squeeze the spinach well and steam briefly. Season with salt and nutmeg.
Spread approx. 4 tablespoons of minced meat in an ovenproof dish (approx. 20 x 30 cm). Cover with 4 lasagne plates. Cover with half of the remaining mince. Place 4 lasagne plates on top. Spread half spinach and half ricotta sauce on each plate. Pile 4 lasagne dishes on top. Add the rest of the minced meat and cover with 4 lasagne dishes. Put the rest of the spinach and ricotta sauce on top.
Preheat oven (electric cooker: 200 °C/circulating air: 180 °C/gas: see manufacturer). Finely grate the Parmesan cheese and sprinkle over the casserole. Wash the tomato, cut into slices and spread on the lasagne. Bake in a hot oven for about 45 minutes.