If you don’t have prepared pesto on hand, first mix the olive oil with the basil in a blender or food processor. Drain and rinse the beans. Drain the tuna.
Combine the lemon juice with the pesto in a mixing bowl, then add the beans and tuna.
Quarter the tomatoes, spoon out the seeds, and finely chop the flesh. Add the tomatoes to the bowl. Toss the ingre