Grilled Boneless Salmon Steaks with Horseradish Dill Butter

Stir together butter, horseradish, dill, shallot, 1/4 teaspoon salt, and 1/4 teaspoon pepper in a bowl with a rubber spatula until combined well. Transfer butter to a sheet of parchment or wax paper and roll into a 1-inch-thick cylinder. Twist ends of parchment to close, then chill until firm, about 20 minutes.
Prepare grill for direct-heat cooking over medium-hot charcoal.
Put salmon steaks on a cutting board with tail-like flaps nearest you. Halve 1 steak lengthwise by cutting along each side of large center bone. Remove and discard bones (you will end up with 2 thin pieces of fillet). Flip 1 piece over, turning it so rounded thicker portion is nearest you. Push halves together to create a yin-yang presentation, wrapping tail-like flaps around outside of steak. Wrap a piece of kitchen string around outside of steak, going around once or twice, and tie ends.
Brush salmon on both sides with oil, then sprinkle with remaining 3/4 teaspoon salt and 1/2 teaspoon pepper.
Oil grill rack, then grill salmon, covered only if using a gas grill, turning over once and if necessary moving to area with no coals underneath to avoid flare-ups, until just cooked through, 6 to 8 minutes total.
Serve each fish steak topped with a slice of horseradish butter.