1. Blend the vinegar, oil, chopped onion, honey, salt, and mustard until smooth in a blender.
2. Add the papaya seeds and pulse until they look like coarsely ground peppercorns.
3. In a large bowl or on individual serving plates, combine the salad greens with thinly sliced papaya and vegetables of your choice. Drizzle with some of the dressing and toss gently to coat.