In a small bowl mix butter, zest, 1 tsp lime juice, garlic, coriander, salt and pepper to taste. Rub both sides of swordfish steaks with remaining 2 tsp lime juice and season with salt and pepper. Grill swordfish on an oiled rack set 5-6 inches over glowing coals until just cooked through, 3-4 minutes on each side. Alternatively, swordfish may be grilled in a hot well seasoned ridged grill pan over moderately high heat.
Top each swordfish steak with a dollop of coriander lime butter and serve.