Roast sweet potato, pineapple and bean pastries with tahini, chilli and lemon grass dressing

I like having sheets of rolled puff pastry in the freezer. Here I bake rectangles of it to use to sandwich vegetables and a tasty dressing. While not exactly a millefeuille (’thousand leaves’), it’s near enough, give or take 990-odd layers! The dressing may separate if it sits too long, but keep it out of the fridge and stir briskly just before serving. Kumara works as well as the sweet potato, as does sliced pumpkin or squash, and runner beans or asparagus instead of beans. The pineapple m