In a large skillet, cook beef and onion over medium heat until beef is no longer pink, 6-8 minutes, breaking up beef into crumbles; drain. Stir in beans, picante sauce and chili powder.
In a bowl, combine soup and tomatoes. In a lightly greased 2-1/2-qt. baking dish, layer half each of the chips, beef mixture, soup mixture and cheeses. Repeat layers.
Microwave on medium high, uncovered, until heated through and cheese is melted, about 12 minutes. If desired, top with cubed avocado and sour cream.