Bring a saucepan of salted water to the boil and add the macaroni pasta and chopped onion. Cook the pasta according to the instructions on the packet, or until al dente, and the onion is soft. Drain and return to the saucepan.
To the saucepan add the tomatoes, grated cheese (reserving some for the top) and seasoning. Stir until well incorporated.
Transfer the mixture to an ovenproof dish and sprinkle over the reserved grated cheese then lay the sliced fresh tomato on top. Place under a hot grill to melt the cheese and brown the top off. Once golden brown, remove from the heat and serve.
This is quite a different slope on macaroni cheese, I was brought up on this and I find the recommended recipe quite bland but that is a matter of choice.