Whisk together the olive oil, soy sauce, lemon juice, garlic, red pepper flakes, black pepper, ginger, green onion, and salt in a bowl. Add the mahi mahi and toss to evenly coat. Cover the bowl with plastic wrap, and marinate in the refrigerator for 1 hour.

Preheat the oven's broiler and set the oven rack in the middle of the oven.

Remove fish from marinade, and shake off excess. Discard the remaining marinade. Place fish in a large baking dish in a single layer. Broil in the preheated oven until the fish flakes easily with a fork, 10 to 15 minutes. If the fish browns too quickly, cover and baking dish with a sheet of aluminum foil.

Melt 1 1/2 teaspoons butter and 1 1/2 tablespoons of sugar in a skillet over medium-high heat. When the mixture begins to bubble, stir in mango cubes. Cook and stir until mango is tender, about 5 minutes. Serve by placing a mahi mahi fillet over a scoop of hot rice and top with the mango salsa. (Can substitute with Fresh Mango Salsa recipe.)