Peel the onions and use a mandolin to slice the onions very fine.Melt the butter in a large pot.Add in the onion, and cook until transparent (about 10 minutes, stirring occasionally so the onions don't burn.Add boiling water to the pot, so it comes up about level to the top of the onions.Stir in the breadcrumbs, and quickly stir in the egg yolks (these ingredients are used to help thicken the soup).Stir in the nutmeg.Cover the pot and let simmer for about 10-15 minutes, before serving.