To prepare garlic, cut off the top third of the head. Drizzle with 1 T olive oil and then wrap tightly with aluminum foil. Roast in oven 45min to an hour until very soft.
Place sliced carrots in a baking pan large enough so they sit in a single layer. Drizzle the remaining oil on them and stir so that they are covered. Put the pan of carrots in the oven with the garlic. Give them a stir every 20 minutes or so to prevent sticking.
While carrots and garlic are roasting, heat butter in a large skillet over medium low heat. Saute onions for about 15 minutes stirring occasionally til soft and translucent. Turn off the heat while you wait for the carrots to finish roasting.
Once garlic is soft, remove from oven, unwrap the foil, and set aside to cool.
Turn up the oven to 350 and continue roasting carrots for 15 more minutes.
Now put your carrots in the pan with the onions and add 2 cups of stock. Bring to a simmer over medium heat and simmer for about 15 minutes.
While this simmers carefully squeeze out the bulbs of garlic and place them in a blender. They should pop right out.
Add carrot and onion mixture to the blender as well and blend until smooth.
Pour soup back into the skillet and gently heat to a simmer, adding more stock as needed to reach the consistency you want. Now add your cumin and salt, tasting to see if more is needed.
Ladle into bowls, top with a dollop of sour cream and enjoy!