Let steaks stand at room temperature 30 minutes. With a mortar and pestle or bottom of a heavy skillet coarsely crush peppercorns and in a small bowl combine with salt. Pat steaks dry and rub seasoning onto both sides of steaks, pressing to adhere.
Grill steaks on an oiled rack set 5 to 6 inches over glowing coals 7 to 9 minutes on each side, or until an instant-read thermometer registers 130 degrees F. for medium-rare. (Alternatively, steaks may be broiled on rack of a broiler pan under a preheated broiler about 4 inches from heat.) Transfer steaks to a platter and let stand 10 minutes.
Serve steaks sliced with the horseradish-mustard butter.
In a bowl, blend together the butter, horseradish, mustard, and shallot with a fork. Season with salt and pepper. On a sheet of plastic wrap, form the butter into a log about 4 inches long. Chill the compound butter until firm, at least 30 minutes or freeze until ready to use.
Yield: 6 servings