For the relish heat oil in a sauté pan on medium high heat. Add onion and garlic; cook and stir on medium heat for 3 minutes or until softened. Add half of the grapefruit segments and red pepper; cook and stir for 30 seconds. Remove from the heat. Stir in the honey basil salt and cayenne pepper. Set aside.
Grill oysters flat-side up over medium high heat for 3-5 minutes or until oysters begin to open. Discard any oysters than do not open. Remove oysters from the grill and remove the top shells. Loosen oysters from the bottom shells with a paring knife.Top each oyster with 5 mL of the relish and 5 mL of the remaining grapefruit segments. Grill oysters again for 3-5 minutes or until heated through. Sprinkle with parsley. Serve immediately.