Heat oven to 220C/200C fan/gas 7. Line a very large rectangular baking sheet with baking parchment. Unravel the roll of pastry and carefully lift on top. Sprinkle over the ground almonds and scatter the plums to make an even layer. Sprinkle with the flaked almonds, then the sugar.
Bake for 18-20 mins until the pastry is golden and the plums are tender. Allow to stand for 10 mins to cool a little, then sprinkle with a little more sugar and serve.