In a medium bowl, whisk together olive oil, minced garlic, ground cumin, ground coriander, smoked paprika, ground turmeric, ground cinnamon, salt, black pepper, and freshly squeezed lemon juice. Mix thoroughly until the marinade is smooth and well-blended.

Coat the chicken thighs generously with the marinade, ensuring each piece is fully covered. For best results, allow the chicken to marinate for at least 15 minutes at room temperature or, ideally, refrigerate overnight.

Slice the red onion into thin rings and layer them evenly at the bottom of the slow cooker.

Place the marinated chicken thighs on top of the sliced onions, and pour any remaining marinade over the chicken.

Cover the slow cooker with its lid and set it to low heat. Let the chicken cook for approximately 4 hours.

Once the chicken is cooked, carefully remove it from the slow cooker and shred it using two forks, or slice it into bite-sized pieces.