With a sharp knife, cut peel and outer membrane from grapefruit. Working over a strainer set over a bowl, cut between inner membranes and fruit to release segments into strainer. Squeeze juice from membranes into bowl. Discard peel and membranes.
In a large bowl, combine oil, lime juice, and grapefruit juice. Add salad greens and mix gently to coat. Divide evenly among four plates and top with grapefruit segments, avocado, almonds, and green onions. Add salt and pepper to taste.