1 large cauliflower (about 1.3–1.4kg/3–3lb 3oz), leaves and stalk trimmed away, cut into small florets
1 onion, roughly chopped
2 tbsp finely chopped fresh flatleaf parsley, plus extra to garnish (optional)
900g/2lb potatoes, peeled or well scrubbed
4 spring onions, trimmed, white parts and green parts separated and sliced
1 tsp English mustard powder
70²µ/2½´Ç³ú plain flour
salt and freshly ground black pepper
1 tbsp vegetable oil
150ml/5fl oz vegetable stock
¼ tsp ground white pepper
70²µ/2½´Ç³ú butter
150g/5½oz mature cheddar, grated, plus 2 tbsp extra for topping
750ml/1¼ pint milk
25g/1oz Parmesan, finely grated
4 tbsp single cream