Look for very small on-the-vine tomatoes for this. Just don't use plum tomatoes, because they won't sit up straight. This appetizer should be served as soon as it's made so the guacamole doesn't discolor.
12 small (2- to 2 1/2-inch) tomatoes
1 tbsp. olive oil
1 tbsp. fresh lime juice
3 firm-ripe Hass avocados
2 tbsp. finely chopped onion
1/2 tsp. finely chopped cilantro
2 tsp. minced fresh jalapeño chile, including seeds
1/2 tsp. salt
1. Cut a thin slice from stem end of each tomato and scoop out and discard seeds with a small spoon. Carefully scoop out pulp to form a tomato shell, then finely chop pulp.
2. Drizzle inside of each tomato shell with oil and lime juice and let stand at room temperature while making guacamole.
3. In a bowl, coarsely mash avocado with a fork. Fold in chopped tomato pulp, onion, cilantro, jalapeño, and salt.
4. Spoon guacamole into tomato shells, mounding it up.