Mix the onion, garlic, parsley, lemon juice, salt and pepper and marinate the tournedos in the mixture for one hour. Remove the steaks and pat them dry with paper towels. Reserve the marinade.
Using a heavy frying pan large enough to hold the meat comfortably, heat the butter, which should cover the bottom of the pan generously. Brown the meat over high heat for 2 to 4 minutes on each s