Clean the spinach, wash thoroughly several times and drain. Peel onions and garlic and dice finely. Heat the oil in a large pot. Fry the garlic and half of the onions.
Add the spinach, pour on 100 ml of water, cover and allow to collapse for 3-4 minutes. Season with salt, pepper and nutmeg. Place on a sieve and drain. Cook pasta in boiling salted water according to package instructions.
Put the strained tomatoes, remaining onions and dried herbs in a pot. Season with salt, pepper and sugar. Heat and simmer on low heat for 6-8 minutes. Drain the pasta and let it drain.
Wash the fish and pat dry, cut in half once. Season the tomato sauce once more and mix with the pasta. Spread the noodles in a flat, greased casserole dish. Place the fish on top, season with salt and pepper.
Mix the cheese with the spinach and spread on the fish. Cover and bake in the preheated oven (electric cooker: 200 °C/ convection oven: not suitable/ gas: level 2) on the lower rack for approx. 10 minutes. Cook for another 6-8 minutes without lid.
Serve garnished with basil.