Prepare the Steak: Grind peppercorns coarsely. Season steaks with salt and crushed peppercorns, pressing them firmly into the meat.
Cook the Steak: Heat 2 tbsp. oil in a Dutch oven. Cook steaks until browned on each side. Add butter and thyme sprigs, cooking until the internal temperature reaches 135°F. Let the steaks rest, then dice into cubes. Reserve the cooking fat.
Make the Soup Base: In the reserved fat, sauté the chopped shallot, garlic, and chopped thyme until aromatic. Stir in flour, cook for a minute, then carefully pour in cognac. Gradually whisk in the beef broth.
Simmer the Potatoes: Add the potatoes to the broth and simmer until tender.
Final Touches: Stir in heavy cream and Worcestershire sauce, heating through.
Fry the Shallots: In a separate pan, heat 1/2 cup oil and fry the sliced shallots until golden. Drain on paper towels and season with salt.
Serve: Ladle the soup into bowls, top with steak cubes and fried shallots.