Heat oil in a skillet over medium heat. Add the onions and cook for about 5 minutes until translucent. Push the onions out toward the edges of the pan and turn heat up to medium-high.
Add the ground beef and let it sit and sear for 2-3 minutes. Flip and let it cook another 2 minutes. Break meat apart with a meat chopper or wooden spoon and let it sear again. Continue to chop up and brown meat. Once browned, drain off the grease. (I like to just dump it into a large mesh sieve.)
Add the meat/onions back to the pan along with the water and taco seasoning. Mix together, bring to simmer, reduce heat to low, cover, and let simmer 10-15 minutes or until tender. Place in a bowl and let cool slightly.
In a large bowl, add lettuce, beans, cheese, corn, tomatoes, and olives.
Add the Doritos, meat, avocados, and dressing just before serving. Toss until evenly coated in dressing and serve.