Strain the yogurt in a yogurt strainer or coffee filter for 15 minutes. Mince the cucumber, and place into a mesh strainer to drain for 15 minutes. Squeeze any excess liquid from the cucumber, then place into a small mixing bowl. Stir in the yogurt and dill. Cover, and refrigerate until ready to serve.

Heat the olive oil in a skillet over medium heat; stir in the garlic, and cook until fragrant, about 5 minutes. Set aside about 1/4 of the onion slices to use on the sandwiches, and add the remaining onion to the skillet; cook until soft, about 5 minutes more. Add the ground beef and oregano, and cook until the meat is no longer pink and is beginning to brown. Increase heat to medium-high, drain any grease from the beef, then stir in the celery, spinach, and mushrooms. Cook and stir until the celery has softened and the meat has nicely browned, about 5 minutes more.

To assemble the Scyros, divide the meat among the warmed pitas. Top each sandwich with some onion slices, banana pepper rings, and tomato wedges. Dollop with the Scottziki sauce to serve.