900g/2lb frozen cauliflower or broccoli, or a mix of both
1 medium onion, sliced
1 tsp mustard (optional)
2 tbsp plain flour
salt and black pepper
1 tbsp butter
200g/7oz mature cheddar, coarsely grated
400ml/14fl oz milk (any variety)
300g/10陆oz shredded gammon or ham (see recipe tips)