Thaw fish if frozen. In a small mixing bowl combine onion powder, garlic salt, red pepper, basil, white pepper, thyme, black pepper and sage. Brush both sides of fish with melted butter (butter); coat with seasonings.
If using a charcoal grill, remove grill rack and place an unoiled 12 inch cast iron skillet directly on hot coals. If using a gas grill, turn to high and place skillet on the grill rack. Preheat the skillet 5 minutes or until a drop of water sizzles.
Add cooked fillets to skillet. Carefully drizzle about 2 teaspoons of melted butter over the fish. Grill fish 2 1/2 to 3 minutes or until blackened. Turn fish and drizzle with 2 teaspoons of the melted butter. Grill 2 1/2 to 3 minutes more or until blackened and fish flakes easily when tested with a fork. Drizzle with any remaining butter.