Sort the spinach, wash and drain well. Peel onion and cut into fine cubes. Peel garlic and chop finely. Heat oil in a pan. Fry the spinach, garlic and onion for about 4 minutes, turning them over. Season to taste with salt, pepper and nutmeg. Take out the spinach and put it in an ovenproof dish.
Wash the fish, dab dry, cut in half lengthwise and season with salt and pepper. Spread fish with pesto and spread tomatoes on top. Roll up the fillets and place them on the spinach. Cook the casserole in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for approx. 10 minutes. Wash the basil, shake dry and remove the leaves. Remove the casserole from the oven and sprinkle with basil.