Pop the macaroni pasta into a pan to cook. Add salt, onion and garlic and let it simmer for 15 minutes. If you prefer, you can shallow fry your onions and garlic before adding them to your pasta. When your pasta is cooked, use a colander to drain it, and run at least two kettles full of boiling water through the pasta to take away all the starch.
You might prefer to make a roux to make your sauce (ie cook flour and butter until it forms and then add milk slowly until it is a smooth mixture). I tend to put my milk into a pan, pop in the butter and let it heat up to almost boiling, and then take it off the heat to add a mix of cornflour and a little cold milk to thicken my sauce, and put it back on the heat to cook in, just in the same way as I would do to thicken gravies etc.
Put the oven on to heat at around 190C and butter an ovenproof dish. For these quantities, I use a big stone dish that does the job well. you might find you need to split this over a couple of dishes to finish it off in the oven.
Take the white sauce off the heat and add in 300g of cheddar gradually. Put in a handful and stir until it is melted. You can put it back on a low heat while it is melting.
Once the cheddar is all melted, pop the macaroni into the sauce and mix well before pouring into the prepared dish for oven baking.
Sprinkle 100g Grated Cheddar Cheese on the top of your dish and place your halved cherry tomatoes.
Bake for 15 - 20 minutes, until the top begins to show a golden brown colour.