In a large skillet, cook the beef and onion over medium heat, sauteing until the beef is no longer pink, six to eight minutes. Break the cooked beef into crumbles as it cooks, then drain the excess fluid.
Stir the beans, picante sauce and chili powder into the meat and onions.
In a separate bowl, combine the soup, tomatoes and chopped green chiles.
In a lightly greased 2-½ quart baking dish, layer half each of the chips, beef mixture, soup mixture and cheeses, then repeat the layers.
Microwave the nacho casserole on medium-high, uncovered, until it's heated through and the cheese is melted about 12 minutes. If desired, top the dish with cubed avocado and sour cream before serving.
