Place the rosemary leaves and salt into a food processor and blitz until well blended with a rough texture.
Preheat the oven to 200°C. Place the grapes in an ovenproof dish and pour over the balsamic vinegar. Roast until the skins split and the grapes are slightly caramelised.
Place the grapes onto a precooked pizza base, saving the pan juices for serving. Bake the pizza for 5 minutes or until the edges are crisp and charred. To serve, sprinkle over a generous amount of rosemary salt, grapes and cheese. Drizzle over the pan juices and serve warm.