Place the chicken broth, olive oil, lime juice, garlic, jalapeno, salt, cumin, paprika, and pepper into a food processor. Blend until the garlic and jalapeno,if using) are pulverized and the sauce is well incorporated.
Pour the sauce over the chicken and marinate for 1 hour or up to over night. If you are oven roasting, reserve about 1/4-1/2 cup of the marinade for after the chicken is cooked and shredded.
If you are roasting in the oven, preheat the oven to 375 degrees. Place the chicken in a small roasting pan and top with the sliced onions. Cover with foil and roast for 35-40 minutes. Remove and let cool to the touch. Shred the chicken. Either heat the remaining sauce and mix into the chicken and serve immediately. Otherwise, you can cool down and when you are ready to reheat, toss with the reserved marinade and heat through and serve with the chopped cilantro.
If using the slow cooker, lay the chicken in a single layer in the crock pot. Pour the sauce over the chicken and sprinkle with the sliced onion.
Cover and cook on low for 8 hours for best results (or high for 4 hours).
Using two forks, shred the chicken. Add the cilantro to the chicken, and toss again. Serve immediately. ENJOY!