1. In a bowl, mash the butter, parsley, garlic and Cognac; season
with salt and pepper.
2. In a large skillet, heat the 3 tablespoons of olive oil. Add the
leeks and season with salt. Cover and cook over moderately low
heat, stirring occasionally, until tender, 10 minutes.
3. Light a grill or heat a grill pan. Brush the steaks with olive oil
and season with salt and pepper. Grill over high heat, turning
once, until nicely charred outside and medium-rare within, 5 to 6
minutes per side. Transfer the steaks to a carving board and let
rest for 5 minutes. Slice the steaks across the grain and spread
the butter all over the meat, letting it melt in. Serve with the leeks.