Thaw fish, if frozen. Cut fish fillets into 4 serving-size portions, if necessary. Rinse fish; pat dry with paper towels; set aside.
In a large skillet heat 1 tablespoon of the cooking oil. Cook mushrooms, green or yellow sweet pepper, and onion in the hot oil for 5 minutes or just until tender. Remove vegetables with a slotted spoon; set aside.
Add the remaining cooking oil to skillet. Add fish fillets. Cook over medium heat for 8 to 10 minutes or until fish flakes easily with a fork, turning once.
Spoon the cooked vegetables over fish fillets. Top with salsa. Cover and cook over low heat about 2 minutes or until heated through. Garnish with fresh oregano, if desired. Makes 4 servings.