Layer the chopped onion, green beans and mushrooms in the bottom of a 5- to 5-1/2-quart slow-cooker. Arrange the chicken thighs on top. Sprinkle with thyme, salt and pepper. Pour in chicken broth.
Cover pot; cook on HIGH for 4 to 5 hours, or on LOW for 9 to 10 hours, or until the chicken and green beans are tender.
At end of cooking, stir together flour and the 1/2 cup water in small bowl until smooth. Stir 1 cup of cooking liquid from slow-cooker into flour mixture until smooth; stir into slow-cooker. Cook, covered, on high for 10 to 15 minutes or until thickened. Stir in sherry, heavy cream and pimiento; cook, covered, on high power another 5 minutes. Stir in the Parmesan cheese.
While chicken mixture is cooking in step 3 above, cook spaghetti following package directions. Drain.
Transfer spaghetti to a platter and top with chicken mixture. Sprinkle with parsley, if desired.