BRIE AND CAVALO NERO TOASTED SANDWICHES

100g Waitrose Blueberries • 2 tbsp balsamic vinegar • 1 rosemary sprig • 1 tsp olive oil • 1 garlic clove, peeled and bashed • 100g cavolo nero, stalks removed and leaves cut into strips • Pinch salt • 4 large slices No.1 White Sourdough Bread • 15g unsalted butter 1 tbsp wholegrain mustard • 100g French mature ripe brie, sliced