Finely grate the parmesan. Whisk with eggs. Season with salt and pepper. Sort and wash the spinach. Peel garlic and chop finely.
Heat the butter in a large ovenproof pan. Add the garlic and dripping wet spinach and let it collapse briefly while turning. Season with salt, pepper and nutmeg.
Preheat the oven grill. Pour the egg and parmesan mixture over the spinach in the pan, mixing gently. Crumble the gorgonzola over the spinach. Let it simmer for about 10 minutes at low to medium heat.
Cut the mozzarella into slices. Cover the frittata with it and continue to bake under the grill for about 5 minutes until the cheese has melted. Serve. Serve with toasted bread and salad.