Cut the peeled onion in half rings and sauté with the bacon in olive oil until translucent. Whisk the eggs with salt, pepper, nutmeg and Parmesan, pour over the onions and cover the frittata over a moderate heat. When the surface starts to thicken, turn the frittata and finish baking. To turn, turn it over on the lid or on a large plate and slide it back into the pan.