Peel and finely dice the onions. Peel potatoes, wash thoroughly and grate coarsely. Squeeze out the liquid firmly between your hands. Mix potatoes, approx. 1/3 of the onions, mustard and crème fraîche.
Wash the fish and pat dry, season with salt and pepper. Place on a baking tray lined with baking paper and spread the potato and mustard crust evenly over the fish.
Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) for approx. 35 minutes. In the meantime, clean and wash the spinach and drain well. Clean, wash and halve the tomatoes.
Heat the oil in a large pot, fry the tomatoes and remaining onions in it while turning, season with salt and pepper and remove.
Add the spinach to the pot and let it collapse while turning. Add the tomato and onion mixture again and season to taste with salt, pepper and nutmeg. Remove fish from the oven. Arrange vegetables and fish on plates.