Preheat oven to 425F/220C.
Pulse the date and walnut pieces together until they are broken into crumbs.
Lay the puff pastry flat on a clean surface, lightly floured if the pastry is a little sticky, and flatten any folds.
Sprinkle over the date-walnut mixture and distribute evenly. It won't cover, but you do want to avoid large clumps. If there are any larger pieces of date, break them up as you go.
Sprinkle over most of the sugar in a thin layer.
Fold the pastry in from two opposite sides to halfway to the middle. Then fold each side again to the middle, with a slight gap left. Fold one half on top of the other so you end up with 6 layers in all.
Ideally, transfer to a baking sheet and chill for 20min or so to help the pastry firm up before slicing but you can also cut immediately if relatively firm. Cut slices around ½in/1.25cm thick. Carefully transfer the slices to a lined baking sheet, cut side up, with plenty of space between as they will expand as they cook. Sprinkle over any leftover sugar in any folds that look a little short on filling.
Bake for approx 15min until golden brown. The dates/sugar may go relatively dark but you don't want them to burn so do watch towards the end. Reduce the temp if they are getting dark but the pastry hasn't browned yet. Allow to cool then enjoy. They'll keep for a couple days in a sealed container, though they will lose a little crispness.