Bring a large pan of salted water to the boil. Add the pasta, stir well, bring back to the boil and then cook, uncovered, for a couple of minutes. Tip in the cauliflower florets, bring back to the boil again and cook for a further 8-10 minutes until both the pasta and the cauliflower are tender but still firm to the bite. Drain well. Preheat a hot grill.
Add the crème fraîche, mustard and all but a good handful of the cheese to the pasta pan. Stir over a low heat until the cheese just starts to melt. Tip the pasta and cauliflower into the cheese sauce and gently stir together. Season and transfer to a flameproof dish.
Scatter the tomatoes over the top followed by the rest of the cheese and some pepper. Grill for about 5 minutes until browned and bubbling.