Grease a large casserole baking dish, then set baking dish aside. (I used a 4-quart oval baking dish for this recipe.)
In a skillet, brown the sausage along with the onion, until the sausage is cooked half-way through, then add the garlic, and continue cooking until the sausage is completely cooked through. Place in colander to drain off any excess grease.
While the sausage is draining in the colander, start cooking the macaroni, and until it's al dente' (about 10 minutes). I do add a bit of vegetable oil to the pot of water before it comes to a boil.
While the macaroni is cooking, I add the cooked sausage back to the skillet, and over a low flame, then I add both cans of diced tomatoes, and the can of cheddar cheese soup. Stir until the soup is incorporated, then add in 1 cup of the shredded cheese, and stir till it's almost mixed in, but not completely melted in.
Once the macaroni is done cooking, and has been thoroughly drained in the colander, add it and the sausage/tomato/cheese mixture to the prepared casserole baking dish; gently stir till all ingredients are mixed together.
Top the casserole with the remaining cup of shredded cheese, then bake it (uncovered) in a 400º oven for 30 minutes.