Thinly slice the onions, slowly sweat in the butter, barely coloring them. Add and brown the flour. Add half of the cold water, bring to a boil stirring. After a few minutes, add the rest of the water with the stock cube. Season with salt, pepper and nutmeg, boil for 10 minutes. Meanwhile, slice the French bread and toast it in the oven. Pour the soup into 6 ovenproof bowls. Add slices of toasted French bread and generously sprinkle with grated Swiss cheese. Slowly brown in the oven, as if a gratin. Prepare 6 glasses, pour the port and the egg yolk in each one of them. The guests will all lift the crust on the surface of their soup, add the port and egg, stirring with a fork and trying not to break the grilled cheese crust. A ready made onion soup can be the base to this recipe.
