Thaw fish, if frozen. Rinse fish steaks; pat dry with paper towels. Set aside. In a medium skillet, cook onion and garlic in hot oil until tender. Stir in tomato, salt, and pepper. Bring to boiling; reduce heat. Simmer, uncovered, for 15 minutes. Stir in 2 tablespoons of the basil.
Meanwhile, combine melted butter and the remaining basil; brush over one side of the halibut steaks.
Lightly coat the unheated rack of a broiler pan with cooking spray. Place fish, brushed side up, on broiler pan. Broil 4 inches from heat for 8 to 12 minutes or until fish begins to flake when tested with fork, turning once.
Season fish to taste with additional salt and pepper. Serve with tomato mixture. Makes 4 steaks.