In a medium bowl, whisk together the Greek yogurt, lemon juice, minced garlic, olive oil, dried oregano, salt, and black pepper until smooth and well combined.

Place the chicken thighs in your slow cooker in a single layer if possible, then pour the marinade evenly over the top, making sure each piece gets coated.

Cover and cook on low for 6 to 7 hours or on high for 3 to 4 hours until the chicken reaches an internal temperature of 165°F and shreds easily with a fork.

Using two forks, shred the chicken directly in the slow cooker, letting it soak up all the cooking juices. If there's excess liquid, drain some off or let it sit uncovered for 10 minutes to thicken.

Warm your pita bread in the oven or microwave, then fill each one with the shredded Greek chicken, fresh cucumber slices, diced tomatoes, red onion, crumbled feta, and a generous dollop of tzatziki sauce.