1. 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 dry. Reserve the marinade.
2. Using a heavy frying pan, heat the butter. Brown the steaks over high heat for 2-4 minutes on each side, depending on the degree of doneness desired. Remove to a warmed serving platter. Cook the reserved marinade in the butter remaining in the pan, 2-3 minutes. Pour over the steaks and serve.