Stir-fry beef with onion, garlic, chili powder and salt over medium high heat until beef is browned and completely cooked (about 8-10 minutes). Drain, if necessary, and set aside.
For the dressing, mash the avocado with a potato masher until smooth. Stir in green onion, water, lime juice, sour cream and salt.
In a shallow 12 cup (3 L) glass bowl, layer tomatoes, then lettuce, followed by the cooked ground beef mixture, cheese and kidney beans. Top with the prepared avocado dressing. Garnish with grated cheddar cheese and green onion.