Season beef heavily with salt and pepper on all sides.
Sear the beef until browned all over in a large pot. Add bone broth until covered. Bring to a light simmer on the stove then cover with a lid.
Braise beef in the oven at 350°F for 2.5 hours.
While the beef is braising, slice the onions and caramelize them in butter in a separate pot until nicely browned and aromatic. This should take at least 2 hours.
Remove the cooked beef from the pot and save the remaining bone broth.
Deglaze the onion pan with cognac and dry white wine. Cook until the liquid is 80% reduced.
Strain the reserved bone broth into the onions. Tie the thyme, bay leaves, and whole black peppercorns in a cheesecloth to make a bouquet garni, and add it to the pot.
Season with 1/2 Tbsp salt and 1/4 tsp black pepper. Simmer semi-covered for 2 hours.
Remove the aromatics from the soup. Shred the cooked beef and add it back into the pot.
Pour the soup into oven-safe bowls. Top with cubes of crusty bread and a generous amount of grated Gruyère cheese. Bake at high heat until the cheese is melted, bubbly, and browning all over.
Cook it hands-free.
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