Place onion in a medium bowl and cover with cold water. Set aside. Combine avocado, cilantro, lime juice, oil, garlic and salt in a mini food processor. Process, scraping down the sides as needed, until smooth and creamy.
Just before serving, combine salad greens, corn, tomatoes and beans in a large bowl. Drain the onions and add to the bowl, along with the avocado dressing. Toss to coat.
