Layer the pearl onions, button mushrooms, garlic, fresh rosemary and bay leaf in the bowl of a 6 quart slow cooker
. Add the browned chicken to the slow cooker bowl. Mix the balsamic vinegar, chicken stock, tomato paste and brown sugar in a medium bowl or a 4 cup measuring cup and season with salt and pepper. Pour over the chicken and cook on high for 3 hours, testing for doneness and cook another 15 minutes to a half hour until chicken is no longer pink.