Thaw fish, if frozen. Measure thickness of fish. Combine the onion powder, garlic powder, white pepper, cayenne, black pepper, thyme, and salt. Brush fish with some of the melted butter or margarine. Coat fillets evenly on both sides with pepper mixture.
Remove grill rack from grill. Place a cast-iron 12-inch skillet directly on hot coals. Heat 5 minutes or until a drop of water sizzles in the skillet. Add fish to skillet. Drizzle fish with remaining melted butter or margarine. Cook, uncovered, for 2 to 3 minutes per side for 1/2- to 3/4-inch-thick fillets (3 to 4 minutes per side for 1-inch-thick fillets), or until fish flakes easily when tested with a fork. Makes 4 servings.