Sift flour into a mixing bowl. Add egg yolks, sugar, small flakes of butter and salt. Mix to a pliable dough. Chill for 2 hours.

Preheat the oven to 375°F (190°C) Gas 5.

Roll out dough and use to line 9 inch (23 cm) flan ring on a baking tray. Bake blind for 15 minutes.

Remove flan case from oven and reduce heat to 300°F (150°C) Gas 2. Remove foil and beans. Add blackberries to flan and sprinkle with peel.

Whisk egg whites until very stiff. Fold in sugar followed by ground almonds, lemon rind and cinnamon. Put 3 tbsp of the mixture in a piping bag fitted with a large rose nozzle. Spread the remainder over blackberries. Sprinkle edges of flan with flaked almonds and decorate the top with swirls of piped egg white mixture.

Bake for 1 hour 10 minutes. Remove from oven and cut flan into wedges before it cools. When cold, remove flan ring and serve.