1Place mahimahi in shallow dish. Pour lime juice over. Marinate 1 hour, turning fish over occasionally.
2Preheat oven to 350°F. Combine panko and next 3 ingredients in small bowl. Mix in 2 tablespoons butter. Season generously with pepper. Pour remaining 2 tablespoons butter into shallow baking dish. Remove mahimahi from marinade and place in baking dish, turning to coat with butter. Spoon panko mixture atop fish, dividing evenly. Press topping gently to adhere. Bake until fish is cooked through, about 20 minutes. Transfer to plates and serve.