Preheat oven to 190C. Line a baking sheet with baking parchment paper (or use the parchment from the pastry packet). Unroll the puff pastry and cut it in half lengthways. Then cut vertically to make eight squares. Brush the edge of each square with a little milk, then fold the edges over slightly to make a thin border. Spread the pesto into each square, then top with the cheese (if using). Using a small heart-shaped cutter, cut the red peppers into shapes and top each square with some red pepper hearts. Brush the edges of the pastry with a little milk, then bake for 25 minutes or until golden. Serve warm.