Layer the potatoes & cauliflower. Layer potatoes in the bottom of the slow cooker and the cauliflower on top of the potatoes. Add the garlic, milk, chicken broth, salt and pepper and toss to coat, leaving most of the potatoes on the bottom of the pot.
Cover and cook. Cover and cook until fork tender, about 3 ½ to 4 hours on high, or 7 to 8 hours on low, gently stirring 1-2 times during cooking.
Mash the potatoes and cauliflower. When the potatoes and cauliflower are very soft, stir in the butter and sour cream. Mash using a potato masher for a chunkier texture, or blend with an electric mixer for a smoother consistency.
Season to taste. Season to taste with additional salt and pepper, and add additional milk to achieve desired consistency. Top with fresh chives if desired and serve immediately, or cover and set the slow cooker to WARM until ready to serve, adding more milk before serving if necessary.
