Whisk together the marinade by combining olive oil, minced garlic, ground cumin, ground coriander, smoked paprika, ground turmeric, ground cinnamon, salt, black pepper, and lemon juice in a large bowl. Mix well until all ingredients are blended.

Coat the chicken by placing the chicken thighs into the marinade and ensuring they are thoroughly covered. Allow them to sit for at least 15 minutes, preferably overnight.

Layer the onions by slicing the red onion and placing it at the bottom of the slow cooker.

Add the chicken on top of the onions in the slow cooker, pouring any remaining marinade over the chicken.

Cover and cook on low heat for 4 hours.

Shred or slice the chicken from the slow cooker and serve warm.