2 onions, finely sliced
1 medium potato, peeled and cut into cubes
陆 tsp English mustard
450g/1lb plain flour, plus extra for dusting
1 tbsp plain flour
2 tsp baking powder
salt and freshly ground black pepper
陆 tsp cayenne pepper
陆 tsp salt
120g/4陆oz unsalted butter, plus extra for greasing
150g/5陆oz mature cheddar cheese, grated
50ml/2oz double cream
1 free-range egg, beaten, for glazing
1 free-range egg yolk
50ml/2oz whole milk
50g/2oz parmesan cheese (or a similar vegetarian hard cheese), grated