Combine rice vinegar, honey, and water in jar with tight lid; shake until well blended.
Add onion slices to the jar. Shake jar to coat onions in liquid, then refrigerate overnight or for at least 6 hours. If necessary, shake the jar 2-3 times during the 6 hours to be sure the onions marinate evenly.
Grate 2 teaspoons of orange peel.
Combine orange peel, oil, vinegar, and honey in a small jar with a lid.
For each serving, arrange endive and/or romaine leaves in a spoke-fashion on individual plates.
Pare and slice oranges and grapefruit into circles; remove seeds.
Portion oranges, avocado, and grapefruit pieces onto the center spokes of endive and/or romaine.
Lift marinated onions from liquid with a fork and transfer to the top of each salad.
Drizzle with dressing before serving.
Note: Rice vinegar can be found in the Asian or ethnic foods section of grocery stores.