Green Salad with Papaya Seed Dressing

1/3 cup rice vinegar
1/3 cup canola oil
1/2 small sweet onion (such as Vidalia or Maui), chopped, plus 1 large sweet onion, thinly sliced into rings and rinsed with cold water
1 1/2 teaspoons sugar
1/2 teaspoon salt
1/2 teaspoon dry mustard
1 1/2 tablespoons fresh papaya seeds
1 pound mixed salad greens
2 avocados, pitted, peeled, and sliced

1. 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.

2. 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.