In a glass dish, combine the oil and lemon juice, season with salt and pepper; add the fish, cover, and marinate in the refrigerator for 1 hour, turning once.
Oil the grids of the barbecue or a ridged grill pan, heat over high heat until hot. Meanwhile, combine the chopped parsley, red pepper flakes, and butter in a microwave-safe measuring cup; microwave until the butter is melted. Add the marinade from the fish and mix well.
Place the halibut on the grill and cook, basting with the marinade; turning once, until just opaque. Discard any unused marinade.