Ainsley Harriott served up a tasty potato tart with mustard, leeks and mushrooms on The Best Dishes Ever.

The ingredients for the potato pastry are: 100g plain flour, plus extra for dusting, salt and freshly ground black pepper, 1 tsp English mustard powder, 75g chilled unsalted butter, diced, 100g leftover mashed potato and ½-1 tbsp iced water (optional).

For the filling: 2 tbsp olive oil, 400g leeks, trimmed and thinly sliced (225g trimmed weight), 100g chestnut mushrooms, brushed to remove any grit, sliced, salt and freshly ground black pepper, 75g mascarpone, 1 tbsp wholegrain or Dijon mustard, 50g Cornish blue, Stilton or Gorgonzola (or a similar vegetarian blue cheese), diced, pinch crushed dried chillies (optional) and lightly dressed fresh green salad, to serve (optional).