French onion soup from Lyon

Melt 1 T. butter in a heavy skillet. Add 2 onions, finely chopped, and cook for ten minutes over low heat. Add 8 c. chicken stock and bring to gentle boil. Let simmer for 10 minutes. Slice 1 day old baguette (French bread) into thick rounds and place on cookie sheet. Toast until crisp and golden on edges. Place a layer of baguette slices in an overnproof tureen. Sprinkle with grated Gruyère (Swiss) cheese. Alternate layers, ending with cheese. Ladle the soup over the bread and cheese. Place in lowest part of oven and cook for 30 minutes at 350°. In a small bowl, whisk 1 egg, 1/4 c. port, and black pepper. When toping in tureen begins to brown, remove from the oven. Using a ladle, gently push aside the crust and remove a cup or so of soup. Pour the soup into the egg mixture slowly, stirring constantly. Pour the egg mixture back into the tureen under the crust. Bake another 10-15 minutes, then serve immediately.