Peel and cut sweet potatoes into equal pieces approximately 1/4" thick. Combine sweet potatoes, cream, butter, and salt in a saucepan. Cover and cook on low heat for 30-40 minutes or until potatoes are soft and tender. (That's right, no boiling in water first, this braising technique works great with less fuss when it's time to mash them.) Try not to lift the lid to check for at least 30 minutes. If the potatoes are not tender after 30 minutes, stir and replace the lid for 10 more minutes or until tender.
Roast garlic by cutting the top of the bulb off, exposing the tops of the garlic cloves; place in a small oven safe dish, drizzle oil over garlic bulb and roast in oven at 375° for 30 minutes or until garlic is fragrant, lightly browned, and soft. Allow to cool until easy to handle. To remove the roasted garlic from the bulb, squeeze the bulb at the bottom releasing the cloves out the cut end. Cloves should pop right out.
Once the potatoes are soft and tender, add roasted garlic directly into pan of sweet potatoes and smash together with a potato masher for a chunky texture, or use a hand mixer to whip to desired consistency. Season with pepper and serve in a warmed bowl or keep warm in the oven in a covered casserole dish until ready to serve.
Celeriac root can be sliced and cooked along with the sweet potatoes to add even more complexity to the flavor of this dish.