8-10 boneless, skinless chicken thighs
1-2 tablespoons extra virgin olive oil
1 16 ounce package frozen white pearl onions
8 ounces white button mushrooms, quartered
5 garlic cloves, minced
2 stems fresh rosemary
1 bay leaf
¾ cup balsamic vinegar
1 cup chicken stock
3 tablespoons tomato paste
¼ cup brown sugar
kosher salt and freshly ground black pepper
1 tablespoon butter
Season chicken thighs with kosher salt and freshly ground black pepper. Heat olive oil in a large frying pan over medium high heat and brown thighs in batches for about 5 minutes on each side or until golden brown. Remove from the pan and set aside.
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.
Transfer the chicken to a serving dish and cover with a lid or foil. Separate the liquid from the vegetables, and add the vegetables to the chicken. Add the liquid to a sauce pan and cook over medium heat, at a low boil, for about 5 minutes or until reduced by half. Add the butter and whisk until melted and sauce has thickened.
Garnish the chicken and serve immediately with rice, mashed potatoes or other grain and balsamic sauce.