Half fill a large saucepan with water and bring to the boil. Add the macaroni, return to the boil and cook for 10-12 minutes, or according to the packet instructions, stirring occasionally until tender.
Meanwhile, prepare the sauce. Put the butter, flour and milk in a non-stick saucepan and bring to a gentle simmer, whisking continuously with a silicone whisk. (Using a silicone covered whisk will prevent your pan becoming scatched.)
Cook for 3 minutes or until the sauce is thickened and smooth, stirring continuously. Add roughly two-thirds of the cheese and season with salt and black pepper. Cook for 2-3 minutes more, while stirring, and add a little more seasoning if necessary. Preheat the grill to its hottest setting.
Drain the pasta in a colander and return to the pan. Gently stir in the hot cheese sauce until thoroughly combined. Tip the macaroni cheese mixture into a warmed shallow flameproof dish (a lasagne dish is ideal) and spread to the corners.
Arrange the sliced tomatoes on top. Sprinkle with the rest of the cheese and add a couple of twists of ground black pepper. Cook under the grill for 3-5 minutes or until the tomatoes are hot and the cheese is lightly browned.