Marinated Onions: Peel and thinly slice red onion. Combine 1/4 cup rice vinegar, 1/4 cup honey and water in jar with tight lid; shake well until blended. Add onions to jar. Shake to coat onions in liquid, then refrigerate at least 6 hours. If necessary, shake jar occasionally to be sure onions marinate evenly.
Dressing: Combine oil, 1/4 cup rice vinegar, 1 tablespoon honey and orange zest in a small jar with a lid.
Salad: Pare and slice oranges; remove seeds. Slice avocado. For each appetizer-salad serving, arrange endive and/or romaine on individual plates. Portion oranges, avocado and grapefruit slices onto the center spokes of the lettuce. Lift marinated onions from liquid with a fork and transfer to the top of each salad. Drizzle with dressing before serving.