Preheat the oven to 350 degrees. Put the celeriac and garlic in a large saucepan and cover with water. Simmer over high heat until the celeriac is tender, about 15 minutes. Drain the celeriac and garlic and place them on a baking sheet.
Bake the celeriac and garlic for about 10 minutes to dry them. Remove from the oven, return to the saucepan, and mash well with a potato masher. Add the mascarpone and butter and mix thoroughly. Season to taste with salt and pepper and servewarm.