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Heat the oven to 180C/160C fan/gas 4. Put the gammon, skin-side up, in a deep roasting tin, then pour over 1 litre water. Scatter over the peppercorns, onion, carrots and bay leaves, then cover tightly with foil and roast for 2 hrs.
While the ham is in the oven, cook the cauliflower in a large pan of boiling salted water for 3-4 mins until just cooked, then drain well. Leave to steam-dry in the colander.
Remove the ham from the oven, take off the foil and set aside on a large dish or board until cool enough to handle. Strain the liquid into a jug and set aside. Using a sharp knife, cut away the string and skin to leave a thin, even layer of fat. Use the tip of the knife to score the ham fat into a diamond pattern, if you like.
To make the cheese sauce for the cauliflower, melt the butter in a shallow saucepan and stir in the flour to make a sandy paste. Slowly pour in 500ml of the reserved ham stock to make a silky sauce, then add the cream. Season with a grating of nutmeg and a grinding pepper, then taste to see if it needs more salt. Bring to a simmer and stir gently for 5 mins, then take off the heat and stir through all the cauliflower and most of the cheese. Will keep chilled for up to 24 hours.
Heat the oven to 220C/200C fan/gas 7. Spoon the cauliflower cheese into a large roasting tin and bake for 20 mins until the sauce is bubbling and the cauliflower is tender.
Heat the grill to medium, and nestle the ham, fat-side up, in the middle of the cauliflower cheese. To glaze, brush the ham fat with the mustard, then scatter over the sugar (don’t worry if some goes on the cauliflower). Scatter over the remaining cheese (avoid getting any on the ham) and grill for 10-15 mins until the ham is glossy and browned and the cauliflower cheese is bubbling and golden. Leave to rest for 5 mins before serving.