Onions cooked to a golden brown in beef stock and cognac.
Sauté onions and some salt in melted butter over medium- low heat until they caramelize and even char a bit about 35-45 min.
Add cognac and increase the heat to high for about 5 minutes to cook off some of the alcohol. Add the beef stock and thyme bring to a boil,. Reduce heat, cover, and simmer gently for 1 hour. Add salt and pepper, to taste.
Set oven to broil. Slice French bread into 3/4-inch slices and place on a cookie sheet, toast slices on one side until golden brown. Remove from oven.
Ladle soup into ovenproof bowls, float the bread, toasted side down, and cover with the Gruyere and Parmesan cheeses. It can even spill over the sides. Place ovenproof bowl on a baking sheet and broil until cheese melts. Remove from oven and sprinkle a bit more grated Parmesan cheese over it. Serve immediately.