Preheat the oven to 220C (200C fan). Put the lemon juice, sugar and cinnamon in a large bowl and mix together.
Add the plums and toss through until well coated. Taste your filling and add another teaspoon of sugar, if needed.
Unroll the sheet of puff pastry and, keeping it on its backing paper, cut the sheet into sixteen equal rectangles.
Using scissors, cut the whole sheet in half, including the paper. Set one half aside.
On the other half, share the plums equally between each pastry rectangle. Leave any excess juice in the bowl.
Grab the other half of the puff pastry you reserved earlier and lay it on top of the plums, paper side up. Peel the paper away.
Press the tips of your fingers all the way around each pastry to form eight parcels.
Separate them out a little to give you room to press around the edges with a fork to seal. Trim the edges with a sharp knife to neaten up, if you wish. Transfer the parcels on a large baking tray lined with baking paper.
If you have any little holes in the pastry, you can repair them with a little off-cut of pastry press gently over the holes.
Brush all over the top of each pastry with the remaining juice from your plum bowl – this will give the pastry a great colour and flavour.
Make two small slits on the top of each parcel with a sharp knife.
Bake for 15-20 minutes until well-puffed, reddish-golden and cooked through.
As there's fresh fruit in this plum pastry recipe, the finished pastries are best stored in the refrigerator.Transfer them to a sealed container within 2 hours of making and place in the refrigerator, where they'll keep for up to three days.