If necessary, clean the spinach, wash it thoroughly and drain it in a colander. Peel garlic and onion and chop finely. Heat the fat. Fry garlic and half onion cubes.
Pour in broth. Add spinach in portions and steam for about 3 minutes. Season to taste with salt, pepper and nutmeg. Wash the fish and dab dry. Season with salt and pepper. Sprinkle with lemon juice.
Pour the spinach with the stock into a small casserole dish. Place the fish on the spinach and cook in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for 15-20 minutes. Meanwhile clean, wash and chop the tomato.
Grate cheese. Wash the oregano, dab dry and remove the leaves. 5 minutes before the end of the cooking time, sprinkle cheese, oregano, tomato and remaining onion cubes on the fish and finish cooking. Remove and serve immediately.