Place mushrooms, onions and garlic in a 5- or 6-qt. slow cooker. In a small bowl, whisk remaining ingredients; pour over mushrooms. Cook, covered, on low until mushrooms are tender, 6-8 hours., Freeze option: Freeze cooled mushrooms and juices in freezer containers. To use, partially thaw in refrigerator overnight. Microwave, covered, on high in a microwave-safe dish until heated through, stirring gently and adding a little broth or water if necessary.