Cut off the bottoms and tops, cut into quarters and peel. Rinse in cool water if there is any remaining dirt or debris.
Cut into 1/2-inch thick dice.
Heat the olive oil in a 4-quart saucepan over low heat just until it shimmers.
Add the celeriac, garlic, salt and pepper and cook, stirring frequently, just until it begins to soften, approximately 5 minutes. Increase the heat to medium-high and add the water. Bring to a boil and cook until the celeriac is tender and easily pierced with a fork, approximately 20 minutes.
Drain the celeriac through a colander and return to the pot. Using a stick blender, puree until no lumps are present, approximately 1 minute.
Place the butter and heavy cream into a microwave proof bowl and heat just until the butter is melted, approximately 45 seconds.
Add the cream and butter and continue to puree with the stick blender for another minute.
