Roast the beef bones

• Preheat the oven to 400°F (200°C).
• Coat the beef bones with tomato paste on all sides and place on a baking tray.
• Roast for 40-50 minutes until deeply browned and caramelized.

Simmer the beef stock

• Add the roasted bones to a large pot or pressure cooker with the onion, carrots, celery, thyme, parsley, peppercorns, bay leaves and water.
• Pressure cook for 1 ½ hours, or bring to a boil, reduce to a simmer, cover and cook for 4-6 hours.
• Strain the stock and set aside.

Caramelize the onions

• Heat the butter and olive oil in a large pot over medium heat.
• Add the sliced onions, season generously with salt and mix to coat with the fat.
• Cover and cook, stirring often, for 1-1 ½ hours until deeply caramelized, removing the lid after 30 minutes and deglazing with a splash of water if needed.

Build the soup

• Add the flour to the caramelized onions and cook for 2-3 minutes.
• Add the white wine and cognac and reduce for 4-6 minutes until most of the liquid is gone.
• Add the beef stock, beef bouillon and Worcestershire sauce, plus the bay leaves, rosemary and thyme tied with twine.
• Bring to a boil, reduce the heat to low and simmer for about 30 minutes.

Make the crostinis

• Preheat the oven to 375°F (190°C).
• Toss the baguette slices with melted butter, thyme, salt and pepper in a bowl.
• Lay them on a baking tray and bake for 15-20 minutes until crisp.

Broil and serve

• Season the soup with salt and pepper and mix the Gruyère with the Parmesan.
• Ladle the soup into oven-safe bowls, add a generous layer of cheese, two crostinis, then more cheese.
• Broil on a baking tray for 3-5 minutes until melted and golden, then rest for 5 minutes.
• Sprinkle with chives and enjoy.