Preheat the oven to broil. Butter a baking dish. Saute the pancetta in a large pan until crispy and golden brown. Remove pancetta, leaving the fat in the pan. Add the butter and let it melt. Add the flour, and cook for one minute while stirring. Gradually add the milk, beating well after each addition to avoid lumps. Add the salt, pepper and mustard, then the grated cheese. Cook until the cheese has melted, then add in the cooked pasta and mix together.
Transfer the macaroni to the baking dish and top with the remaining handful of grated cheese. Arrange the halved cherry tomatoes on top then grill for 5 to 10 minutes until golden brown and bubbling. Allow to rest for a few minutes before serving.