1. Prepare the turnips and potatoes for cooking. Rinse, peel, then cut into large cubes. Bring a pot of water to a boil and add the cubes of turnips and potatoes. Boil until everything is tender, 10 to 15 minutes depending on the size of the pieces.
2. Drain the mixture and return to the pot. Add in the butter, heavy cream, garlic powder, sage, and salt. Mash until everything is well blended and smooth.