1kg/2lb 4oz big floury potatoes, peeled and cut into large chunks
5–6 spring onions, finely sliced
1 tsp English mustard powder or English mustard
salad, pickle and mustard, or steamed green vegetables
salt and freshly ground black pepper
25g/1oz butter, diced, plus extra for greasing
225g/8oz mature cheddar, grated
1 large free-range egg, beaten
250ml/9fl oz milk