SEASON THE FILETS generously with salt and pepper. In a medium cast-iron skillet over medium-high heat, melt 2 tablespoons of the butter with the olive oil and garlic. When the butter begins to bubble, add the filet mignon pieces. Tip the skillet slightly and baste the tops with the butter and olive oil. Cook until deeply browned on the bottom, about 3 minutes. Flip and continue cooking and basting on the other side until brown, about 3 minutes. Turn the pieces on their sides and sear the edges until brown. Remove from the skillet and let the steak rest while you prepare the leeks and sauce.