In a large dutch oven, combine the potatoes, heavy cream, milk, garlic cloves sage, and thyme. Bring to a gentle boil over medium heat and cook 20-25 minutes, until the potatoes are fork-tender.
Drain the potatoes, reserving all of the cream. Discard the thyme and sage. Press the potatoes and roasted garlic cloves through a potato ricer. Then add them back to the warm pot. Alternately, you can add the potatoes back to the pot and mash with a potato masher.
Over low heat, add 1 1/2 cups of the reserved cream and the butter. Continue to add more of the cream until your desired consistency is reached. Season the potatoes with salt and pepper.