Cook Mac and Cheese according to box, strain, and set aside (don't add any ingredients to pasta yet).
Steam Cauliflower 6-7 minutes or until fork tender.
While Cauliflower is cooking heat olive oil in skillet and sautee chopped onions and garlic for 2-3 minutes or until slightly browned.
Add cauliflower, butter, onions and garlic, milk, and cheese packet from mac and cheese to blender and puree until smooth. I used the sauce setting on my Blendtec Blender.
Salt and Pepper to taste.
Notes
You may need to increase milk to obtain desired consistency. I sprinkled mine with parsley just to make it prettier ; )