To make garlic confit, heat oil and garlic in a saucepan over low heat. Cook on low heat for one hour. Do not brown the garlic. Add salt and pepper. Move to blender and mix (or mash by hand) into a chunky paste.Rub garlic confit over skin of fish. To intensify flavor, cover and refrigerate for 24 hours before cooking—but it can be cooked right away.Fire up grill to medium heat. Grill grapefruit halves face down for 15 minutes until dark brown so that the sugar of the grapefruit caramelizes. If the grill has a top shelf, leave the grapefruit on the top shelf face up to absorb some smoke.Add fish to grill. Squeeze one half grapefruit juice over fish.Grill over medium heat 6 minutes. Flip, squeeze remaining grapefruit half over fish, and cook an additional 5 minutes.Serve whole on platter. The meat should scoop out easily.