In a small bowl, combine the lemon juice, sea salt, oregano, garlic powder, and black pepper. Grease a 4- to 6-quart slow cooker with the olive oil. Add the flank steak to the slow cooker. Pour the lemon juice mixture over the steak and stir to combine. Cover and cook on LOW for 7 hours.

In a bowl, combine the yogurt, garlic, red wine vinegar, dill, sea salt, and black pepper.

Set aside.

Remove the beef from the slow cooker. With 2 forks or a shredder, shred the beef.

Divide the beef among the pita breads. Top the beef with the tomato, cucumber, and red onion. Serve with the tzatziki sauce.