Step 1Scoop out the avocado flesh and place it in a food processor. Add the onion, lime juice, yogurt, jalapeno and salt and process until smooth.
Step 2Put the mixture into a shallow rectangular glass dish, cover and freeze, stirring the mixture with a fork every 30 minutes to maintain a creamy consistency, until the glace is scoopable, about 2 hours, 45 minutes.
Step 3Serve two small scoops in each of six highball glasses. Add a sprinkle of diced tomato, a drizzle of olive oil (about one-half teaspoon in each glass) and three to four cilantro leaves.