Preheat oven to 210℃/375℉. Line two baking trays with baking paper.
Wash, core and slice your pears (or apple) thinly. Toss in a little of the lemon juice.
Brush the defrosted puff pastry squares with the jam, leaving space at the outer edge. Fold and lightly pinch together pastry edges to make a border.
Lay your fruit however you please around the pastry.
Brush pastry and fruit with melted butter.
Sprinkle sugar over pastry, including edges and fruit. Use as much or as little as you like.
Bake for about 25 minutes but keep an eye on it. The melted butter will cause the pastry to brown quickly. The fruit will release juices but don't worry unless it starts swimming, in which case you can gently spoon some out.
To ensure bottom is cooked, try gently to see if you can moving the tart across the baking paper. Cut each pastry into four pieces. Serve with cream or ice cream.