Remove the extra papery skin from outside the garlic head, keeping the inner layer of skin that covers the cloves. Cut a very thin slice off the top of the garlic head (about 1/4 inch) just to expose the top of the cloves.
Drizzle the exposed garlic cloves generously with olive oil. Wrap the garlic heads tightly in aluminium foil (if you don't like baking with foil, first wrap them in parchment paper then foil to avoid the foil touching the garlic directly. Place on a baking sheet or in a loaf pan in the hot oven for 30-40 minutes or until the cloves are browned and softened. Set the roasted head of garlic aside to cool while you prepare your cauliflower.
For the cauliflower:
Bring a pot of water (a few inches of water) to a boil and fit with a steamer basket. Place the cauliflower florets in the steamer basket, then tightly cover the pot and steam for 10-15 minutes or until cauliflower is fork tender.
Place the steamed cauliflower (in batches, half at a time) to the bowl of a food processor and process on high until cauliflower has pureed into a mash. Add the sour cream, parmesan, salt, pepper, and nutmeg is using, and process further until cauliflower is creamy.
Squeeze about 1/2 the roasted garlic head into the cauliflower mash and stir or process further to combine. Taste and adjust for seasoning.