Season the steaks generously with kosher salt on both sides. Place the peppercorns in a small bowl or zip-lock bag and crush them coarsely with a mortar and pestle or the back of a spoon. Press the peppercorns into both sides of the steaks, coating them evenly.
Heat 2 tablespoons of olive oil in a skillet over medium-high heat. Once the oil is hot, add the steaks and cook for 3 minutes per side, or until nicely browned.
Flip the steaks and add the butter and rosemary sprigs to the pan. Tilt the pan and spoon the melted butter over the steaks for 1-2 minutes, basting them continuously. Cook until the steaks reach an internal temperature of 135°F (57°C) for medium-rare. Transfer the steaks to a cutting board and let them rest for 5 minutes, tented with foil to keep warm.
Pour out any excess fat from the pan, leaving the cooking oil in the pan. Add the chopped shallots and cook for 2 minutes, until softened and translucent.
Add the garlic and cook for 30 seconds, until fragrant. Stir in the flour and cook for 1 minute, stirring constantly, to make a roux.
Whisk in the cognac or brandy and cook for 1 minute, scraping up any browned bits from the bottom of the pan.
Pour in the beef broth and bring the soup to a simmer. Add the potatoes and cook for 15-20 minutes, or until the potatoes are tender.
Reduce the heat to low and stir in the heavy cream and Worcestershire sauce. Season with salt and pepper to taste.
While the soup simmers, heat the remaining 1/2 cup of oil in a separate skillet over medium heat. Add the thinly sliced shallots and fry until golden brown and crispy. Drain the shallots on paper towels and season with salt.
To serve, divide the soup among bowls and top each bowl with sliced steak and crispy shallots.
Enjoy!