Using a medium saucepan, bring water to a boil and cook macaroni according to package directions. 8 oz. are half of a 1 lb. box of pasta. Double the recipe if you want to cook the entire box.

While the noodles are cooking, shred the cheese using a cheese grater and slice the tomatoes in half. Reserve a little of the cheese to the side if you plan to sprinkle it on top and bake the dish before serving.

Drain the noodles and set aside.

Returning to the medium saucepan, melt the butter and stir in the flour. Cook and stir over medium high heat for about a minute. Add in the cup of milk and continue cooking and stirring until the sauce thickens, just a couple of minutes more. Add in the grated cheese and stir until it melts in.

Add the pasta back into the pot along with the onions, tomatoes and green chiles. Stir until everything is well combined and the pasta is evenly coated with cheese.

From here, you can serve immediately or transfer the contents to a baking dish. If you choose to bake it, add the reserved cheese on top and bake at 400 degrees for 10-15 minutes, or until the cheese melts and browns slightly on top.