Cut green peppers in half crosswise to form shells; carefully remove core and seeds. Bring a large saucepan of water to a boil over high heat. Add peppers, cut side up. Cover and boil for 3 minutes or until tender-crisp. Using a slotted spoon, remove peppers and turn upside down to drain. Refrigerate until ready to use.
In a bowl, combine mayonnaise, yogurt, lemon zest, lemon juice, salt and pepper. Add tomatoes, tuna, chopped lettuce and green onions; fold gently to coat. Spoon into pepper shells, piling high. Refrigerate for at least 2 hours, until well chilled, or for up to 8 hours.
Place each pepper on a lettuce leaf.