Whole baked cauliflower cheese

Why go to the trouble of chopping cauliflower? Roasting it whole is super-simple to prep and makes an impressive addition to a roast dinner

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1 large cauliflower (about 1kg) 50g unsalted butter 50g plain flour Large pinch of sweet smoked paprika 600ml whole milk 2tsp English mustard 170g mature cheddar, gruyere, Comté or a mixture ½ bunch of chives, finely chopped