First let the raisins soak in warm water.
Wash the apples, peel them, remove the core and cut them into cubes, then sauté them in a non-stick pan with butter, brown sugar and a few drops of lemon juice for a few minutes, just long enough to soften them slightly and flavor.
Transfer the apples to a bowl and add the coarsely chopped walnuts, the crumbled biscuits (for a less sweet strudel, replace them with breadcrumbs), the drained and gently squeezed raisins, cinnamon to taste and the jam, and mix.
Unroll the puff pastry, arrange the filling in the center, then distributing it over the entire surface, leaving 1-2 fingers free along the edges.
Fold the 2 short sides towards the center.
Then close the strudel by folding one of the two long sides on itself until it reaches the other side, then seal the edges well.
Brush with a little jam, make some cuts on the surface (to let the steam out, so that the dough does not break) and cook for about 25-30 minutes in a preheated static oven at 200 ° C.
The apple strudel with puff pastry is ready: let it cool, then sprinkle with icing sugar and serve.