Chop celery, red onion, and parsley into small pieces.
Flake drained tuna into a mixing bowl.
Add Greek yogurt, Dijon mustard, lemon juice, garlic powder, salt, and pepper to the tuna, then stir well.
Mix in chopped vegetables, parsley, and pickle relish; stir to combine.
Toast bread slices to desired crispness.
Spread mashed avocado evenly on each slice of toast.
Top avocado toast with the prepared tuna salad.
Optionally, add arugula, drizzle balsamic glaze, sprinkle flaky salt, or add a squeeze of lemon.