Open the tuna cans and drain all the water thoroughly. Place the drained tuna into a medium-sized mixing bowl and use a fork to flake it into small pieces.
Add the Greek yogurt, Dijon mustard, diced celery, red onion, and fresh dill to the bowl with the tuna. Stir everything together until the tuna is evenly coated and the ingredients are well distributed.
Sprinkle in the salt and black pepper. Give it one final stir. Taste a small amount and adjust the seasoning if you prefer a bit more salt or extra mustard for zing.
Divide the baby spinach or mixed greens between two serving bowls. Scoop a generous portion of the tuna mixture onto the center of the greens. Arrange the sliced avocado and cucumber around the tuna.