Melt butter in medium saucepan over medium-low heat. Stir in shallot and jalapeño. Cook, stirring often, 4 minutes or until shallot is soft.
Stir in broth, juice concentrate, and ginger. Increase heat, and boil gently 5 minutes or until reduced to 1 cup. Stir in mango and rum; simmer 2 minutes. Combine cornstarch and water, and stir into mango mixture. Bring to a boil, and boil 1 minute. Remove from heat, and stir in lime juice and salt; keep sauce warm.
Sprinkle fish with salt and pepper. Heat a large, heavy ovenproof skillet over medium to medium-high heat. Add 2 tablespoons oil. Place fillets, seasoned side down, in skillet; cook 1 to 2 minutes or until golden brown. Flip fillets, and put pan in oven. Roast at 425° for 4 to 5 minutes or until fish flakes with a fork. Place fillets on plates, divide sauce evenly over fish, and sprinkle with cilantro. Serve with rice, if desired.