In a bowl, combine the softened butter, crushed black peppercorns, garlic powder, and lemon zest. Mix well until fully incorporated. Place the compound butter on a sheet of parchment paper or plastic wrap. Roll it into a log shape and twist the ends to seal. Refrigerate until firm, about 30 minutes.

In a skillet, heat 1 tablespoon of olive oil over medium-high heat. Add the sliced leeks and season with salt and pepper. Cook, stirring occasionally, until the leeks are caramelized and slightly burnt, about 10-12 minutes. Stir in the chopped rosemary and cook for an additional minute. Remove from heat and set aside.

Pat the filet mignon steaks dry with paper towels and season both sides generously with salt and black pepper. In a large skillet, heat the remaining tablespoon of olive oil over medium-high heat. Once hot, add the steaks and sear for 4-5 minutes on each side for medium-rare, or until desired doneness is reached. Use tongs to flip the steaks.

In the last 2 minutes of cooking, add the burnt leeks to the skillet with the steaks, allowing them to heat through and meld with the flavors of the meat.

Remove the skillet from heat. Let the steaks rest for about 5 minutes. Slice the compound butter into rounds and place a slice on top of each steak. Serve the leeks alongside the filet mignon, garnished with fresh rosemary sprigs.