The Apple Strudel Filling
Heat the butter in a frying pan over medium–high heat. At the first sizzle, add the apple and sugar. Sauté for 3 minutes. Add the lemon zest, almonds and vanilla and continue cooking until the apples are lightly browned on all sides. Blend the cornflour in with the lemon juice and then add the entire mixture to the apples. Cook for another 5 minutes, stirring. Allow to cool.
The Cherry–Ricotta Strudel Filling
In a mixing bowl, whisk together the ricotta, sugar, cornflour and egg whites. When the mixture is smooth, warm it over low heat in a saucepan for 5 minutes to thicken it a little. Allow to cool. Toss the cherries in the flour, then shake them a little to remove the excess.
Preheat the oven to 200°C (400°F/Gas 6). Combine the breadcrumbs, sugar and cinnamon if making the apple strudel, or the breadcrumbs, sugar and vanilla bean seeds if making the cherry strudel. Lay the first sheet of filo on a baking tray lined with baking paper. Brush it with melted butter and sprinkle with the breadcrumb mixture. Place another sheet on top and repeat the process with the remaining sheets of filo pastry and breadcrumb mixture, reserving a little mixture for the top. For the apple strudel, spread out the apple filling in a sausage shape along one long side of the sheets of filo and roll up to enclose the filling. For the cherry–ricotta strudel, spread the ricotta filling over the entire surface of the filo sheets, leaving a 3 cm (1¼ inch) border around the edges. Place two rows of the lightly floured cherries on the ricotta filling, one about one-third along the width of the filo sheets, the second two-thirds along. Roll the filo sheets up widthways to form a sausage.
Brush the surface of the strudel with the remaining melted butter and sprinkle with the remaining breadcrumb mixture. Bake for about 22 minutes. Allow to cool and serve in slices.