Method:
Preheat oven to 200 C
In a bowl, toss the rhubarb with cinnamon, 1 tbsp flour and 3 tbsp sugar.
Line a baking sheet with a piece of baking parchment.
Roll out the puff pastry on a floured surface to approx. 20 x 30cm, then cut into quarters and place on the baking sheet.
separately, rub together the remaining flour, sugar, butter and oats to make a rough crumble mixture.
Divide the rhubarb mixture between the pastry quarters, leaving a 1cm rim on each side.
Sprinkle the oat mixture over, then fold and pinch each corner to keep the filling in.
Bake for 20-25 minutes, then serve warm, best with a scoop of vanilla ice cream

Source: BBC Good Food