Elegant Chicken Cutlets with Brandy Mushrooms and Pearl Onions

4(6-oz.) skinless, boneless chicken breast halves Kosher salt, black pepper 2 T. flour olive oil ¾ c. frozen pearl onions, thawed and drained 8 oz. quartered button mushrooms 2/3 c. brandy 1 c. chicken stock 1 T. cornstarch 1 T…