In a small mixing bowl, whisk together the olive oil, lemon juice, minced garlic, cumin, paprika, turmeric, coriander, cinnamon, cayenne, salt, and black pepper.
Place the chicken thighs into the basin of a slow cooker.
Pour the prepared marinade over the chicken and toss by hand or with tongs to ensure each piece is thoroughly and evenly coated.
Scatter the sliced onions over the top of the chicken pieces.
Secure the lid and cook on the low setting for 5 to 6 hours, or on the high setting for 2.5 to 3 hours, until the internal temperature reaches 165 degrees Fahrenheit.
Remove the chicken from the slow cooker and shred or slice into strips using two forks or a carving knife.
Optional Technical Step: To replicate the charred texture of a vertical rotisserie, spread the shredded chicken onto a baking sheet and broil on high for 3 to 5 minutes until the edges are crispy.