Bring 11/2 pints (850 ml) water to the boil in a large pan with 1 teaspoon salt, add the macaroni and boil without a lid, according to the instructions on the packet.
Meanwhile, melt 2 oz (50 g) of butter in a saucepan, add the flour and mustard, and make up a white sauce with the milk, cooking for about 6 minutes before adding 3 oz (75 g) of the grated cheese to melt gently into it, together with a seasoning of salt and pepper and a few gratings of nutmeg. Then in a little butter in a small frying pan, soften the onion, bacon and mushrooms together for about 6 minutes.
Now drain the macaroni in a colander, combine it with the sauce and the bacon, onion and mushroom, then place the whole mixture in a buttered 2 pint (1.2 litre) baking dish. Sprinkle the rest of the cheese on top, plus the pieces of tomato. Finish off by placing the dish under a hot grill until golden brown and bubbling.
Note: if you want to make this in advance, you can reheat it in a high oven, gas mark 6, 400°F (200°C) for about 15-20 minutes.