Prepare vinaigrette: Zest lemon, then squeeze and drain juice. Blend lemon juice, zest, shallot, honey, dill, mustard, pepper, and olive oil together in a blender or mix by hand in a small bowl.
Combine drained tuna with red onions and prepared dressing. Mix.
For each sandwich, split open the pita pocket. Place lettuce and tomato slices in the pita. Top with prepared tuna salad.