Heat the grill to its highest setting and bring a large pan of salted water to the boil. Cook the pasta for 4 mins, then add the cauliflower for a further 8 mins. Drain, reserving 100ml of the cooking water.
Meanwhile, melt the butter in a medium pan over a low heat and stir in the flour. Slowly pour in the milk, stirring constantly until it begins to thicken. Bubble for a few mins, then remove from the heat. Add the cheese, the reserved cooking water and seasoning to taste.
Tip the pasta and cauliflower into a large casserole dish and stir through the cheese sauce. Mix together the garlic, breadcrumbs and thyme, and scatter over the top. Grill for 2-3 mins until bubbling.