Clean, wash and drain the spinach. Peel onion and garlic and chop finely. Heat the oil in a saucepan. Sauté onion and garlic in it. Add the spinach, cover and let it collapse. Season with salt, pepper and nutmeg.
Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Whisk milk and eggs. Season with salt and pepper. Rinse fish, dab dry, cut into 4 pieces. Sprinkle with lemon, salt lightly.
Divide the spinach into four ovenproof dishes (or one large dish). Place 1 fillet on each. Pour egg milk over them. Grate cheese over it. Bake in the oven for about 20 minutes.