Using sharp knife, make three 1/4-inch-deep, 2 1/2-inch-long diagonal cuts in skin of each fish fillet.
Arrange fillets in single layer, skin side up, in 15x10x2-inch glass baking dish.
Pour Sherry and 1/4 cup olive oil over; sprinkle with freshly ground black pepper.
Cover and refrigerate at least 4 hours and up to 6 hours, turning occasionally.
Preheat oven to 425°F.
Combine butter and garlic in heavy medium saucepan.
Cook over medium heat until butter melts and garlic begins to brown, about 12 minutes, stirring occasionally.
Discard garlic clove.
Add panko to butter (mixture will bubble); cook until golden brown, about 3 minutes.
Add lemon juice; remove from heat.
Stir in pimentón.
Season sauce to taste with salt and pepper.
Cover to keep warm while preparing fish.
Remove fish from marinade and drain.
Sprinkle fish with coarse salt.
Heat remaining 1 tablespoon oil in large ovenproof skillet over medium-high heat.
Add fish fillets, skin side down, and cook until lightly browned, about 1 minute.
Turn fillets over; place skillet in oven and roast until fish is just opaque in center, about 2 minutes.
Spoon Roasted Sweet-and-Sour Onions into center of 6 plates.
Top each serving with 1 fish fillet. Spoon sauce over and serve.