Brine the bones and roast them to render the marrow fat.
Toast the baguette slices in the oven with the rendered marrow fat.
Caramelize the onions slowly over medium-low heat.
Make the soup base by adding flour, tomato paste, brandy, and stock, then simmer for 30 minutes.
Add the bouquet garni (herbs tied in leek greens) and simmer for 15-20 minutes.
Assemble the soup by ladling it into oven-safe crocks, topping with the toasts and grated Gruyère cheese, then broiling until browned and bubbly.
Garnish with chopped chives before serving.
