Preparation:
1. Prepare guacamole: In large bowl, combine avocados, onion, lime juice and salt. With fork or potato masher, coarsely mash everything together.
2. Place 3-inch ring mold in center of serving plate. Evenly fill bottom half of mold with 1/2 cup guacamole.
3. Arrange 2 ounces of tuna chunks on top of guacamole.
4. Carefully remove mold. Repeat with remaining guacamole and tuna.
5. To serve, garnish with cilantro sprig; serve with tortilla chips.