Add the drained cauliflower and chicken mixture into an ovenproof dish. Season.
Put the milk and flour into a medium sized saucepan. Bring the mixture to the boil, then turn it down to simmer and whisk it thoroughly until you have a smooth and thickened white sauce. Stir in the mustard.
Add 75g of the cheese and again whisk until a smooth cheese sauce is formed. Season to taste.
Pour the sauce into the oven proof dish over the cauliflower and chicken, sprinkle with the remaining cheese and place onto the middle shelf of your oven.
Bake for approximately 25 – 30 minutes until the cheese has browned slightly and is bubbling.
Remove from oven and allow to cool a little before serving. Then serve with a green salad.
Notes
If the casserole comes out too watery, just use a slotted spoon to remove portions of the casserole.