Combine all seasoning mix ingredients in a small bowl.
Rinse chicken with cold water and pat dry. Brush chicken with olive oil and rub seasoning into chicken.
Roll up balls of foil and place in the bottom of the slow cooker with 2 thick slices of onion. Place chicken on foil, breast side up so it is slightly lifted off the bottom of the slow cooker.
Cover and cook on low 7-8 hours or high 4-5 hours. (165 degrees internal temp)
Remove chicken and place on a small pan. Broil 3-4 minutes to crisp skin (optional). Rest 10 minutes before slicing.
Remove foil and onion from slow cooker and turn onto high. If required, add broth to create about 2 cups of liquid.
In a small bowl, combine 4 tablespoons cornstarch with 4 tablespoons water to create a slurry.
Whisk about ⅔ of the cornstarch into the drippings/broth and let cook on high. Depending on the amount of liquid you may need to add more of the cornstarch slurry to reach desired consistency. Let cook at least 5 minutes before serving.
Taste and season with salt & pepper.