Put the cauliflower in a pan of boiling water and simmer for 5-6 minutes.
In a bowl, mix together the creme fraiche, ¾ of the cheese and mustard (if using)
Drain the cauliflower well and put in a bowl with the creme fraiche mixture and mix well.
Put in a baking dish and top with the rest of the cheese.
Put in a preheated oven at 190°C/375°F/Gas 5 and bake for 20 minutes.
Make a lower fat version of this by using reduced fat creme cheese and cheddar.
Add some extra flavour with a spoonful of mustard and a pinch of nutmeg.
Make sure you don't overcook the cauliflower. You want it to still be a little tender when you put a knife in to it (about 5-6 minutes par-boiled). If it gets too mushy, then it will be watery when it goes in the oven and can cause the sauce to split. You will need to make sure it is drained properly too.