Preheat grill to high heat (400° to 450°). Preheat a 12-inch cast-iron skillet on grill 15 minutes.
In a small bowl, stir together paprika, thyme, onion powder, salt, garlic powder, black pepper, mustard, and red pepper. Pour melted butter in a shallow dish. Dip each fillet in butter, turning to coat. Sprinkle both sides of fillets with spice mixture; pat gently to coat.
Add olive oil to skillet (oil should smoke); place fish in skillet. Cook, covered, until browned, 3 to 4 minutes. Turn, and cook, covered, until fish flakes easily with a fork, 3 to 4 minutes more. Carefully remove skillet from grill.
Place lemons cut side down on grill. Cook, covered, until charred, 3 to 4 minutes. Serve with fish.
Recipe by Southern Cast Iron at https://southerncastiron.com/blackened-grouper/