Steam cauliflower in microwave or vegetable steamer until tender.
Meanwhile, set a small nonstick skillet over medium high heat and melt butter. Add in onion and garlic and sauté until tender and onions become translucent, about 2-3 minutes.
Transfer cauliflower, onions and garlic into blender or food processor. Add in milk and yogurt. Puree until smooth. If sauce is too thick, add in a bit of water or vegetable broth until desired texture is reached.