Season both sides of the chicken thighs with salt and pepper.
Heat the vegetable oil in a non-stick skillet over medium-high heat. Sear the chicken on both sides to brown—no need to cook through, the slow cooker will finish the job.
Transfer the browned chicken thighs to the slow cooker.
In a food processor, combine the green onion, yellow onion, garlic, ginger, lime juice, jalapeno pepper, soy sauce, ketchup, brown sugar, thyme, cloves, nutmeg, and allspice. Pulse until smooth and well combined.
Pour the sauce over the chicken thighs in the slow cooker, making sure to coat them evenly.
Cover the slow cooker and cook on LOW for 8 hours or on HIGH for 4 hours, or until the chicken is cooked through and tender.
Serve the chicken with the desired sides and garnish with the cilantro if desired.