Cook ground beef and onion until meat is browned and onion is tender; drain. Stir in beans, tomato sauce, taco seasoning mix and 3/4 cup cheese. Sprinkle 2 1/2 cups corn chips in bottom of 8-inch square baking dish. Cover with beef mixture. Bake at 350 degrees F for 20 to 25 minutes.
Spread sour cream over chili. Top with remaining corn chips and cheese. Bake 3 to 4 minutes longer.