Notes: You can assemble this salad (step 1) and make the dressing (step 2) up to 4 hours ahead. Cover salad and chill; cover dressing and let stand at room temperature.
Thinly slice avocado and arrange slices along one side of a wide, shallow bowl; brush lightly with vinegar. Mound lettuce in center of bowl, and arrange tomatoes opposite the avocado.
In a small bowl, mix 3 tablespoons vinegar, olive oil, and 1/2 teaspoon salt.
Pour dressing over salad; mix gently at the table, adding pepper and more salt to taste.
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