Put the pasta into a large saucepan with the cauliflower and cook in gently boiling salted water for 10-12 minutes, until the pasta is tender.
Meanwhile, put the milk, butter and flour into a large saucepan (preferably non-stick) and bring to the boil, stirring constantly with a whisk to give a smooth sauce.
Remove from the heat and stir in about three-quarters of the Parmigiano Reggiano and mustard. Avoid doing this over the heat, as it could make the sauce stringy. Season to taste with salt and pepper.
Preheat the grill and warm a large heatproof serving dish underneath it for a couple of minutes.
Drain the pasta and cauliflower very thoroughly, then tip it into the warm serving dish. Pour the sauce over the top, mixing it in gently. Scatter the breadcrumbs and reserved cheese over the top.
Dot the halved cherry tomatoes over the surface and arrange the pancetta slices on top.
Grill for 6-8 minutes until the pancetta is crispy and the surface is golden brown and bubbling. Serve, garnished with chopped flat leaf parsley.