Trim away any excess fat from the steak or steaks. Press the crushed peppercorns and garlic onto both sides of the steak and let it stand at room temperature 1 hour.
Heat a heavy iron skillet over high heat. Grease the skillet with a piece of the fat trimmed from the steak. Toss in about half the salt. Sear the steaks quickly on one side, then lift out; add the remaining salt to the skillet, turn the steak and sear the other side. Reduce the heat to moderate and cook the steaks as desired. Remove to a warmed platter.
Make the sauce. Pour off fat from the skillet. Melt the butter in the skillet over low heat; add the onion and garlic and cook slowly until soft, stirring often. Add the remaining ingredients and simmer for 1 to 2 minutes. Pour over the steak or steaks and serve.