Set a skillet over medium heat and allow it to get hot before adding the butter and cooking until it just melts. Quickly whisk in xanthan gum until a paste has formed.
Add the milk and whisk more to get out the lumps. Cook 1-2 minutes, until it begins to thicken.
Add the sharp cheddar, mild cheddar, and third cheese (I used asiago) and stir to get the melting process started. Cook for about 5-7 minutes. Cheese mixture will first turn grainy, then should become smoother.
In a bowl, mix together the bag of cauliflower rice (still frozen) and the hot cheese mixture. Transfer it to ramekins or a very small casserole dish and bake, uncovered, for about 30 minutes or until cheese is done to your liking.