Bring a large pan of slightly salted water to the boil then add the macaroni. Cook according to the instructions on the packet (usually about 7-8 minutes in boiling water). While it's cooking, make the Cheese Sauce (and if you decide to cheat and use packet sauce, just don't tell me about it!!)
Peel and chop the onions into small pieces. Fry them gently in the fat (oil) for about 10 minutes, turning frequently, until they become limp and translucent. De-rind the bacon (if necessary) and chop into small pieces. Fry this for just 2 or 3 minutes along with the onions in the frying pan.
When cooked, drain the macaroni and then return it to the pan. Add the onions and bacon from the frying pan and the cheese sauce (which may, or may not, have come from a packet!!). Stir well, then season with salt and freshly ground pepper. Place this mixture into an oven-proof casserole dish then grate (shred) and sprinkle the cheese over the top. Slice the tomato and lay the slices on top of the cheese then put the dish (uncovered) in a preheated moderate oven (Mk 5 - 375ºF - 190ºC) for about 20 - 25 minutes until the cheese is nicely browned on top.
Serve just as it is or with liberal helpings of tomato ketchup.