Add the cut up cauliflower, milk, and water to a stock pot or deep saucepan. Bring to a boil, lower the heat, and simmer, uncovered, for 10 to 15 minutes, or until the cauliflower is easily pierced with a fork or sharp knife. Pour into a colander and let it drain for a full 10 minutes. When the 10 minutes are up, puree the cauliflower, roasted garlic, Greek yogurt, black pepper, and nutmeg in a blender or food processor fitted with a metal blade until silky smooth. Taste and adjust with salt if needed.