For dressing, in a large salad bowl, whisk together the vinegar and cumin. Gradually whisk in olive oil in a slow, steady stream. Add the salt and pepper.
Add the lettuce, tangerines, onion, avocados, and 6 tablespoons of the pumpkin seeds to the salad bowl. Toss lightly to coat with dressing. Sprinkle with the remaining 2 tablespoons pumpkin seeds. Serve at once. Makes 8 to 10 servings.