Add the cut up fresh cauliflower, milk, and water to a stock pot or deep medium saucepan. Bring to a boil, lower the heat, and simmer, uncovered, for 10 to 15 minutes, or until the cauliflower is fork tender or easily pierced with sharp knife.
Pour the cauliflower into a colander and let it drain for 10 minutes. Draining the full 10 minutes is important. You do not want any remaining liquid in the cauliflower when you start to puree it.
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. You can also return the drained cauliflower to the pan and mash it into a puree with a hand blender, potato masher, or -for very rustic texture- a sturdy spoon. Taste and adjust with salt if needed.
If desired, serve with minced green onions on top.
Serve immediately or store in an airtight container in the refrigerator for up to 4 days. For longer storage, divide into labeled single serving size containers and freeze for up to 3 months.