Salad: Chop the lettuce, bell pepper, tomatoes, and green onions. Place in a large bowl, and add black beans, corn, and avocado. Gently stir to combine. Toss with desired amount of dressing. Serve on a platter or on individual plates or bowls.
Dressing: Puree all ingredients in a food processor, blend until smooth. Taste and adjust seasonings if necessary.