Wash the greens well and discard any tough stems. Tear into bite size pieces.
Working over a bowl to catch the juices, peel the grapefruits and oranges and cut between the membranes to remove the sections. Remove the seeds. Place the grapefruit and orange supremes in another bowl, reserving the juice. Add onion to bowl with supremes and mix.
In a small bowl, whisk together the chipotle chiles, vinegar, olive oil, salt, pepper and 2 tablespoons of reserved grapefruit/orange juice.
To serve, pour dressing over fruit, add greens and toss lightly. Arrange on a platter or among 4 salad plates and garnish with avocado slices.