Preheat oven to 215℃/419℉.
Slice the puff pastry into two large triangles (see photo).
Into each triangle place peaches (8-10 slices), then a tsp of butter, 2 tsp of brown sugar and 1 tsb of jam. The aim is to fill the triangle, without overfilling, so whatever fits. It make take a little experimentation.
Add other fruit if using.
Fold pastry over fruit and jam, making a little square, however you can. Pinch pastry well so contents don't escape. At this point, if you judge that that your pastry can take a little more fruit, you can squeeze it in. If contents do escape, never mind, just when they're done drizzle the escaped juice over pastry and pretend that's how you wanted it.
Bake on baking paper on oven tray for approx 25 minutes until golden and bottom has crisped up. Over-baking will make the pastry bottom hard.
When done, sprinkle top with a little extra sugar. Serve warm or cold, with ice cream or cream.