Add honey Dijon mustard, brown sugar, Worcestershire sauce, butter, garlic powder, salt, and water (or chicken broth) to the bottom of your slow cooker. Set to high and stir until butter melts and mixture is smooth.
Place chicken thighs in a single layer and toss to coat in the sauce.
Cover and cook on high for 2 ½ hours or low for 5 hours, until chicken is cooked through (internal temperature should reach 165°F).
Push chicken to the side and stir in green beans. Cover again and cook 30 minutes on high or 1 hour on low, until tender but crisp.
Taste sauce and season with black pepper. Serve hot with white rice, brown rice, or mashed potatoes
