Crumble sausage into a large skillet; add onion. Cook over medium heat until meat is no longer pink; drain well. Add 2 cups tomatoes and picante sauce. Bring to a boil. Reduce heat; simmer, uncovered, for 20 minutes or until most of the liquid has evaporated.
Sprinkle tortilla chips over a 12-in. pizza pan. Top with 2 cups cheese and the sausage mixture; sprinkle with remaining cheese. Bake at 350° for 8-10 minutes or until cheese is melted. Sprinkle with avocado and remaining tomatoes.