In a blender, whirl vinegar, oil, chopped onion, sugar, salt, and mustard until smooth. Add papaya seeds and pulse until seeds look like coarsely ground peppercorns.
In a large bowl, combine onion rings, salad greens, and 3/4 of avocado slices; pour dressing over salad and toss gently to coat. Arrange salad on a large platter and top with remaining slices of avocado.
Note: Nutritional analysis is per serving.