In a small mixing bowl, whisk together the olive oil, lemon juice, garlic, cumin, coriander, paprika, turmeric, cinnamon, salt, and pepper to create a marinade paste.
Place the sliced onions in an even layer at the bottom of the slow cooker basin.
Place the chicken thighs into the slow cooker and pour the spice marinade over the top, tossing the chicken to ensure every piece is thoroughly coated.
Cover the slow cooker and cook on the High setting for 4 hours or the Low setting for 7 hours, until the chicken is fork-tender.
Remove the chicken from the slow cooker and transfer to a cutting board. Shred the chicken into bite-sized pieces using two forks or slice into thin strips.
Optional: For a crispy texture, spread the shredded chicken onto a baking sheet and broil on high for 3 to 5 minutes until the edges turn golden brown.
Drizzle 2 to 3 tablespoons of the residual cooking liquid from the slow cooker over the chicken before serving.