1Preheat oven to 350 degrees F.
Fill a large pot with lightly salted water and bring to a rolling boil over high heat.Once the water is boiling,stir in the macaroni,and return to a boil.Cook the pasta uncovered,stirring occasionally,until the pasta has cooked through,but is still firm to the bite,about 8 minutes.Drain well in a colander set in the sink.
2In a large skillet over medium heat,cook and stir the ground beef with red onion until the meat is browned,about 10 minutes.Break the meat up into crumbles as it cooks;drain excess grease.Mix in the tomato sauce,corn,taco seasoning and chili seasoning;bring to a boil,reduce heat to a simmer,and cook,stirring occasionally,for about 20 minutes.
3Whisk the cheddar cheese soup and milk in a saucepan until smooth,and bring to a simmer over medium-low heat.Whisk in 1 cup of cheddar cheese,1/2 cup sour cream,and garlic powder;season to taste with sodium-free salt and pepper.
4Arrange the cooked macaroni into the bottom of a 10 x 15-inch baking dish,and mix with the soup mixture.Pour the ground beef chili over the macaroni,and sprinkle with remaining cheddar cheese and crushed tortilla chips.Bake in the preheated oven until the casserole is hot and bubbling and the cheese topping has melted,20 to 30 minutes.Serve topped with dollops of sour cream.