Preheat the oven to 400˚F. In a small saucepan, mix together the butter, parsley, garlic, salt, pepper, lemon juice and lemon peel. Set aside.
Pat the mahi mahi fillets dry with paper towels.
On large cast iron skillet, heat the olive oil over medium high heat. Once the oil is hot, add the fish fillet and top with salt and pepper to taste. Cook until browned, around 3 minutes. Flip and turn the heat off.
Place the skillet in the preheated oven for 5 minutes, or until the centre is cooked through.
Meanwhile, heat the small saucepan with the garlic mixture on medium heat, stirring until melted. Pour on the fish.