In a large mixing bowl, combine the ground chicken, finely chopped onion, minced garlic, whisked eggs, whole-wheat breadcrumbs, lemon juice and zest, dried thyme, cinnamon, nutmeg, and salt. Mix until well combined.
Form the mixture into small patties or meatballs to help them hold their shape while cooking.
Optional: Heat olive oil in a skillet over medium heat and sear the chicken patties for 3-4 minutes on each side until golden brown for added flavor.
Place the chicken patties in the slow cooker. Cover and cook on low for 4-6 hours or on high for 2-3 hours, until cooked through and tender.
While the chicken cooks, chop tomato, slice cucumber, and prepare Greek yogurt sauce by mixing yogurt with lemon juice and a pinch of salt if desired.
When the chicken is done, remove from slow cooker and let rest for a few minutes. Warm naan or pita bread, then layer chicken, chopped tomatoes, sliced cucumbers, and Greek yogurt. Squeeze fresh lemon juice over and serve.