Prepare the Bone Marrow: Brine the marrow bones and roast them in a preheated oven at 400°F for 20 minutes.
Slice the Onions: Slice the onions in half and then into thin slices.
Caramelize the Onions: Melt butter in a large pot over medium heat, add onions, and cook until caramelized, about 30-40 minutes.
Make the Soup Base: Add flour and tomato paste to the caramelized onions, stir to combine, and cook for a few minutes.
Deglaze the Pot: Add brandy to deglaze the pot, scraping up any browned bits.
Add Broth and Seasonings: Pour in the beef broth, add fish sauce, Worcestershire sauce, soy sauce, and bay leaf. Simmer for 30 minutes.
Prepare the Toasts: Slice the baguette and toast it in the oven until golden brown.
Assemble the Soup: Ladle soup into crocks, top with toasted baguette slices, and cover generously with Gruyère and Parmesan cheese.
Broil the Soup: Place the crocks under the broiler until the cheese is bubbly and golden brown.
Serve: Garnish with chopped chives and serve hot.