250g/9oz floury potatoes, peeled
2 spring onions, finely chopped
1 tsp English mustard
salt and freshly ground black pepper
1 tsp butter, plus extra for greasing
1 free-range egg, beaten
dash of milk
2 x 320g/11陆oz ready-rolled puff pastry sheet, defrosted and chilled
200g/7oz smoked cheese, grated