Heat the oven to 180‘
Bring a pan of water to the boil, add the cauliflower and cook for 5 minutes
In the meantime, melt the butter in a pan and sauté the onions until they start to golden.
Add the milk, holding a small amount back, mix this with the flour to make a paste.
Once the milk has reached boiling point add the flour paste and stir until thickened
Add the crumbled stock cube, the nutmeg and the mustard.
Combine the cauliflower and the sauce in an oven proof dish, top with the grated cheese and bake for 35 minutes, until the topping has turned golden and crispy