Preheat the oven to gas 7, 220°C, fan 200°C. Arrange the frozen berries evenly in a large ovenproof dish and scatter over 2tbsp caster sugar.
Brush the rim of the dish with a little egg white. Place the puff pastry sheet over the dish, pressing down firmly with your fingers to seal. Using a sharp knife, trim the excess pastry leaving a slight overhang, then make a couple of slits in the lid (this will allow the steam to escape, preventing soggy pastry). Brush with the remaining egg white.
Combine the remaining sugar and the cinnamon in a small bowl and scatter it over the pastry lid. Bake in the oven for about 20 minutes until puffed and golden. Serve with ice cream, if you like.
Using just the egg white for glazing gives the pie a lovely frosted finish once baked.
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