Take a sheet of ready-made puff pastry and place on a baking tray lined with parchment paper. With a sharp knife, cut a 1 cm wide strip
from each side. Brush the edge of the large piece of pastry with water and lay the strips along the edges, to build a little wall
all around the edge. Prick the inside of the pastry all over with the tip of the knife. Brush with beaten egg, sprinkle with caster sugar
and bake in a 200°C oven for 15 – 20 minutes, until golden and crisp and the sides have risen. Remove from the oven and gently push down
the middle if it has risen. Let cool. Put 250g blueberries into a saucepan with 2 Tbs sugar and a squeeze of lemon juice and cook gently
for about 6 – 8 minutes, until the blueberries split and are juicy. Let cool. Whisk 250g mascarpone with 2 Tbs icing sugar, add 200ml
double cream and a tsp vanilla essence and continue whisking until holding soft peaks. Spread the cream all over the pastry, then spoon
over the cooled blueberry compote, and garnish with chopped mint leaves.