1. Preheat oven to 375°. Separate garlic into cloves and peel. Put cloves on an 8- by 12-in. piece of aluminum foil and drizzle with olive oil. Fold foil over garlic and crimp edges to seal. Bake until garlic is soft and starting to brown, about 30 minutes. Set aside.
2. Put potatoes in a large pot and cover with cold water. Bring to a boil and add 1 tbsp. salt. Cook until potatoes are tender when pierced with a fork, about 20 minutes. Drain well.
3. Meanwhile, in a large bowl, mash roasted garlic with buttermilk and demi-glace. Set aside.
4. Put unpeeled potatoes through a ricer into bowl of garlic and buttermilk (discard peels left behind in ricer). Or, as soon as they're cool enough to touch, peel potatoes, put them in bowl with garlic and buttermilk, and mash with a potato masher, a large fork, a wooden spoon, or an electric mixer. (If using a mixer, be careful not to beat them too much or they will become gluey.) Stir in salt and pepper to taste. Serve warm.
Note: Nutritional analysis is per 1-cup serving.