Peel and slice the onion and garlic cloves as thinly as possible. Heat oil and butter in heavy skillet. When butter bubbles, add onions and garlic. Reduce heat to medium-low and cook onions very slowly for 20 to 25 minutes, until they're golden brown. Watch mixture to avoid burning. When onions are golden, sprinkle with sugar, orange juice, salt and pepper. Allow the juice to evaporate for about 1 minute.
While onions are cooking, heat a grill to medium-high. Brush both sides of mahi-mahi fillets with olive oil. Cook fish for 2 minutes directly on grill; rotate the fish one-quarter turn without flipping it. (This gives the fish an attractive diamond pattern from the grill.) Cook for 5 minutes longer on the same side.
Flip fish and cook for 5 minutes longer. Fish is done when it feels firm to the touch. (Cooking time may vary slightly according to thickness of fillets.)
Serve fillets topped with the onion and garlic mixture.
- Recipe courtesy of Fish 4 Fun, at www.fish4fun.com, a Florida-based online fishing and boating resource.