Combine in a small bowl, curry powder, salt, crushed red pepper, and black pepper. Sprinkle curry mixture over turkey; rub in with your fingers. Place turkey in a 3 1/2- to 4 1/2-quart slow cooker. Add half-and-half.
Cover and cook on low-heat setting for 5 to 6 hours or on high-heat setting for 2 1/2 to 3 hours.
Remove turkey to a serving platter, reserving cooking liquid. Cover turkey with foil to keep warm.
If using low-heat setting, turn to high-heat setting. For sauce, combine cornstarch and water in a small bowl. Stir cornstarch mixture into liquid in cooker. Cover and cook about 15 minutes more until thick. Spoon sauce over turkey. Serve.