Place chopped lettuce in a large bowl; layer cucumber, onion, radishes, and avocado on top of lettuce. Sprinkle 1/8 teaspoon pepper over the avocado; refrigerate 15 minutes or up to 1 hour.
Pour vinegar over salad, tossing gently to coat. Gradually add oil, salt, and remaining 1/8 teaspoon pepper to salad; toss well. Sprinkle with sunflower seed kernels.
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