Sprinkle fish evenly with salt and pepper. Set aside.
Melt butter in a large, heavy skillet over medium-high heat. Add onion, and sauté 8 minutes or until onion is tender. Add garlic, and sauté 2 to 3 minutes. Stir in lime juice. Remove from heat.
Arrange fish, skin side down, on a lightly greased rack in an aluminum foil-lined broiler pan. Broil, 5 inches from heat, 7 to 10 minutes or until fish flakes with a fork. Using a wide metal spatula, lift fish from skin, leaving skin on rack. Transfer to a serving dish. Top with onion mixture; sprinkle evenly with toasted almonds and parsley. Garnish, if desired.