Soak cedar in water for several hours before grilling. Rub olive oil over fish then place the fish on cedar plank. Spread sliced onions over top of fish then squeeze lemon over fish season with chopped garlic, salt and pepper. Cook on covered grill until fish is done (edge of filet will break apart when touched).
Recipe by KITCHEN HUI at https://kitchenhui.com/cedar-grilled-mahi-mahi/