Melt butter in a small saucepan over medium low heat; blend in cornstarch and red pepper. Add mango, sugar and sherry (or mango nectar) and bring just to a boil. Reduce heat and simmer 5 minutes or until fruit is soft and the sauce has thickened. Stir in lime juice and zest and cook an additional 1 or 2 minutes.
Rinse the fish and pat it dry. Brush fish with melted butter. Place fish, skin side on grill over medium heat or under a broiler for about 5 minutes.
Turn and baste the top with butter; cook an additional 5 minutes or until the fish flakes easily with a fork.
Transfer fish fillets to a plate. Spoon warm sauce over fish and serve.