Spray a 6.5-quart slow cooker with cooking spray. Lay carrots on the bottom of the slow cooker followed by potatoes.
Remove the neck and organs from the whole chicken. Rinse and pat dry.
Place the chicken in the slow cooker.
Sprinkle with McCormick garlic and herb seasoning and black pepper. Rub all over the chicken. Add in the Brussels sprouts into the slow cooker.
Place the lid on and cook on low for 8 hours.
Once the internal temperature reaches 175°F, carefully removed the cooked chicken.
Grab 1 cup of the stock from the slow cooker and mix with 1-tablespoon cornstarch.
Heat a saucepan on high and add the butter. Melt the butter, then whisk in the stock slurry. Whisk until thick. Add salt and pepper to taste and remove off heat once thick.
Cut the chicken into 6 pieces, serve with vegetables and top with pan gravy.