Preheat a grill to its highest setting. Place the cream and chopped rosemary in a small bowl and lightly whip with a small whisk or fork until it begins to thicken. Set aside. Cut the cheeses in half horizontally and top each of them with a thick smear of the rosemary cream. Place on a plate and refrigerate until needed.
For the relish, place the onion, vinegar and sugar in a small pan