Sift the flour and salt into a bowl, drop in the butter pieces and blend to a crumb texture.

Mix in the sugar. Add the egg yolk and enough sour cream to make a firm paste.

Knead well, divide in two and wrap in plastic film. Chill.

Roll out each piece large enough to fit a 24 cm (9 1/2 in) spring-form tin.

Line the base and sides of the tin with silicone paper, grease well and lay a sheet of pastry on the bottom; scatter over half the breadcrumbs.

Wash and dry the cherries and remove the stones. If using bottled fruit, drain well and dry on paper towels. Mix the fruit in a bowl with the ground walnuts, lemon zest and vanilla sugar, and spread the mixture on the breadcrumb base in the tin.

Sprinkle with the rest of the breadcrumbs and the caster sugar and cover with the remaining piece of pastry. Brush with egg white and scatter the chopped walnuts all over the top. Bake in a preheated oven at 180°C (350°F) Gas 4 for an hour until slightly coloured.

Serve either warm or cold, with whipped cream on the side.