Peeled the oranges and grapefruit. Section the oranges and grapefruit over a small bowl so you can catch the fruit juices; reserve juices.
2
In a large bowl combine the orange and grapefruit sections, avocado slices, cucumber slices, and onion rings; set aside.
3
Measure the reserve fruit juices; add enough orange juice to make 1/3 cup. Combine the fruit juice, vinegar, sugar, salt, and pepper. Pour over fruit mixture; toss gently. Cover and refrigerate for several hours, stirring occasionally.
4
Before serving, use a slotted spoon to remove fruit mixture from the liquid in the bowl. Arrange fruit mixture on lettuce-lined plates.