1. Cut the avocado in half, and, with a teaspoon, remove the seed and any brown tissuelike fibers clinging to the flesh. Remove the skin by stripping it off with your fingers, starting at the narrow, or stem, end (the darkskinned variety does not peel as easily; use a small, sharp knife to pull away the skin if necessary).
2. Dice the avocado and moisten it with the lemon juice to prevent discoloration.In a mixing bowl, combine the avocado, diced tomato and the minced onion. Add theparsley, salt and a few grindings of pepper, and gently mix the ingredients together.
3. With a whisk or fork, beat the vinegar and oil together, then pour it over the avocado-tomaro mixture. Stir thoroughly again, then chill for at least an hour. Serve as a first course in chilled cocktail glasses or as a luncheon salad, arranged on a bed of crisp lettuce.
