Pour beef broth in the bottom of the slow cooker. Place sliced steak and onions on top of the broth.
In a small bowl, mix together olive oil, garlic, oregano, salt, pepper, and lemon juice. Pour on top of meat and place lid on top of slow cooker. Cook on low for 7-8 hours.
About 30 minutes before serving, combine all tzatziki ingredients in a bowl and refrigerate until ready to serve.
To assemble, place meat on a pita and top with shredded lettuce, tomatoes, and sliced cucumbers. Drizzle with Tzatziki sauce.