1 medium cauliflower (around 700g/1lb 9oz), cut into florets, stem sliced and green outer leaves thickly shredded
1 tsp English or Dijon mustard
flaked sea salt and ground black pepper
50²µ/1ΒΎ΄Η³ϊ plain flour (or gluten-free plain flour)
50²µ/1ΒΎ΄Η³ϊ butter
120g/4Β½oz mature cheddar, coarsely grated
500ml/18fl oz milk