Wash and sort the spinach and drain well. Peel and slice the onions. Steam in hot fat until golden brown. Add the spinach and stew in a closed pot until it has collapsed.
Season with salt and pepper. Wash the fish and cut into portions. Bring vegetable stock and wine to the boil and let the fish pieces simmer at low heat for about 8 minutes. In the meantime, wash the tomatoes and lemon.
Cut the mozzarella, tomatoes and lemon into slices. Drain the spinach and place in an ovenproof dish. Spread the rest of the prepared ingredients over it. Wash the thyme, pluck the leaves from the stems and sprinkle over the fish.
Bake in the preheated oven (electric: 225 ° C/ gas: level 4) for about 5 minutes.