Taken from Waitrose recipes
1 onion, sliced
1 cauliflower, halved and cut into florets
300ml whole milk
125g Cheddar cheese
½ tsp Dijon mustard
Heat the butter in a saucepan and add the onion. Slowly cook for 8–10 minutes until soft. Then add the flour, stir together and take off the heat.
Meanwhile, bring to the boil a large pan of water and add some salt. Add the cauliflower, return to the boil and cook for 3–4 minutes so it still has a bite. Drain really well in a colander, shaking it to get it to release all its water.
Add a good splash of milk, beat until it is smooth, then add the rest, again stirring until smooth. Return to the heat, stir occasionally and bring to the boil – it should be a thick sauce – then remove and stir in 75g Cheddar, mustard and seasoning to taste.
Mix the cauliflower with the sauce and transfer to a wide, ovenproof baking dish; the more surface area, the more crispy top you’ll get. Scatter with the remaining cheese and place under the grill for 5 minutes until it is bubbling and golden on top.