Spray 4- to 5-quart slow cooker with cooking spray. In cooker, mix chicken, portabella and shiitake mushrooms, shallots and garlic. Add flour, rosemary, salt and pepper; toss until chicken and vegetables are coated. Stir in broth and mustard until well mixed.
Cover; cook on Low heat setting 8 to 10 hours. Stir well before serving.