Cook the pasta: Bring a large pan of water to the boil. Toss in the pasta and bring back to the boil, then cook for a couple of minutes. Tip in the cauliflower and cook for a further 8-10 minutes until
both are tender.
Make the sauce: Drain the pasta and cauliflower well. Put the crème fraîche, mustard and most of the cheese (keeping back a good handful) into the pasta pan. Stir and warm through over a low heat just until the cheese starts to melt.
Finish the dish: Preheat the grill to hot. Tip the pasta and cauliflower into the cheese sauce and gently stir together. Season to taste. Transfer to a flameproof dish, scatter the tomatoes over the top, then the rest of the cheese and a sprinkling of freshly ground black pepper. Grill for about 5 minutes until brown and bubbling.