Place mushrooms in a 4- or 5-qt. slow cooker. In a small bowl, whisk butter and ranch mix; pour over mushrooms. Cook, covered, on low until mushrooms are tender, about 3 hours. Serve with a slotted spoon. If desired, sprinkle with dill., Freeze option: Freeze 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.