Peel the turnips and cut into cubes, about 1" in size. Place in a large pot and cover with water. Place over high heat and bring to a boil. Season the water with some kosher salt. Boil the turnips until they are tender and can be pierced easily with a fork.
In a small saucepan, combine the cream, sage leaves, and garlic. Bring to a simmer then cover and remove from the heat. Allow to steep until the turnips are ready.
When the turnips are done, drain the water and return them to their pot. Lightly mash the turnips before adding the cream. Remove the sage leaves and garlic clove from the cream. Add the cream to the turnips along with the butter, kosher salt, and pepper.
Mash until you reach your desired consistency. If you prefer a smoother consistency, the turnips can be blended in a food processor. If the turnips are a bit watery, return to the pot over medium heat. Cook gently, stirring to prevent sticking, until the moisture cooks out.