Follow the link above and make instant raspberry jam. Let it cool. (This you can do days ahead.)
Preheat the oven to 200°C.
Lightly whisk the egg in a small bowl.
Roll out the puff pastry to 3mm thickness. Roll the borders inwards to create a dent.
Put the tart onto a non-stick or lined baking sheet.
Using a pastry brush, brush the borders with the egg mixture. ( If you’re using gluten-free pastry, prick the base all over with a fork.)
Spoon the raspberry jam over the base, spread it out up to the border. Scatter the fresh raspberries on top.
Bake for about 20 minutes, until the pastry puffs up and is golden brown.
Let it cool, then cut it into 4 pieces.
Serve with ice cream or thick vanilla custard. Alternatively, just sprinkle some icing sugar all over and serve.