Add the cucumber, celery, scallion, grapes, parsley, and mint. Mix with a fork, breaking up any chunks of tuna, until combined.
To make the dressing, first scoop out the avocado flesh and place it in a medium bowl.
Mash the avocado with a fork.
Add the mayonnaise, lime zest, lime juice, salt, and black pepper to the bowl. Mix well.
Spoon the dressing over the tuna mixture and mix until evenly incorporated. Taste and adjust for salt, pepper, or lime juice, if needed. You can add more mayonnaise if you like it creamier.
Serve on a bed of lettuce and garnished with fresh herbs, if desired.