In a blender or food processor, combine the chopped onion, minced garlic, grated ginger, chopped scallions, seeded Scotch bonnet peppers, ground allspice, ground cinnamon, ground nutmeg, dried thyme, salt, black pepper, soy sauce, brown sugar, lime juice, orange juice, and vegetable oil.
Blend until a smooth marinade forms.
Place the chicken thighs in a large bowl or zip-top bag and pour the jerk marinade over the chicken. Toss to coat evenly. Marinate in the refrigerator for at least 1 hour, preferably overnight.
Transfer the marinated chicken and all the marinade to the slow cooker.
Add the chicken broth to the slow cooker and stir gently to combine.
Cover and cook on low for 6 hours, or until the chicken is tender and cooked through.
Once cooked, remove the chicken from the slow cooker and place on a serving platter.
If desired, pour the cooking juices into a saucepan and simmer over medium heat for 5-7 minutes to thicken slightly, then drizzle over the chicken before serving.