Thaw fish, if frozen. Rinse fish; pat dry with paper towels. Cut into 4 serving-size pieces. In a small bowl combine cilantro, oregano, lime peel, lime juice, oil, garlic, and hot pepper sauce. For relish, in a medium bowl combine drained beans and avocado; stir in half of the cilantro mixture. Cover and chill until ready to serve.
Measure thickness of fish fillets; sprinkle fish lightly with salt and black pepper. Brush the remaining cilantro mixture over all sides of the fish fillets. For a charcoal grill, grill fish on the greased rack of an uncovered grill directly over medium coals for 4 to 6 minutes per 1/2-inch thickness or until fish flakes easily when tested with a fork, carefully turning halfway through grilling. (For a gas grill, preheat grill. Reduce heat to medium. Place fish on greased grill rack over heat. Cover and grill as above.)
To serve, arrange fish on top of relish; garnish with an oregano sprig, if desired.