The secret to successful mahi mahi cookery is to not overcook. Whichever of the following cooking methods you choose, your mahi mahi will be cooked when its flesh becomes opaque but is still moist on the inside.
Rinse fish and pat dry with a paper towel. Place steaks or fillets in baking pan, brush with oil or butter, or cover with sauce made of liquid, herbs and spices, and vegetables. Bake in preheated oven at 400F (200C) until a knife slice in the thickest part reveals the flesh to be opaque but still moist.
Place steaks or fillets directly on greased grill, 4 to 6 inches (about 10 to 15cm) above prepared coals or fire. Baste with butter, oil, or marinade and close the hood of the grill. For a large fillet, place fish skin-side-down on foil. Cook until opaque and moist on the inside, about 6 to 8 minutes for fish less than 1-inch (about 2.5cm) thick, and 10 to 15 minutes for fish larger than 1-inch (about 2.5cm) thick.
Rinse mahi mahi fillets and pat dry with a paper towel. Place fish on a rack above a baking dish. Preheat broiler and adjust oven rack so fish is 3 to 4 inches (about 7.6 to 10cm) from the element. Broil, turning once, until fish is opaque but still moist in the center, about 6 to 10 minutes, depending on the size of the fish.
Rinse mahi mahi, and pat dry with a paper towel. Dredge in flour and seasonings if desired. Shake off any excess flour. Heat frying pan until hot. Add butter or oil, place fillets in the pan, and cook, turning once, until opaque but still moist in the center--about 2 to 10 minutes, depending on the size of the fish.
Bring poaching liquid consisting of water, broth, herbs, and spices to a simmer. Slip in mahi mahi, then cover the pan and keep liquid at a simmer for about 8 minutes per inch (about 2.5cm) of thickness.