How to Make It
In a large saucepan, add the celery root and turnips and just enough cold water to cover; bring to a boil. Stir in 3 tablespoons of the unsalted butter and 2 teaspoons of salt. Reduce the heat to moderate and simmer until the celery root and turnips are tender and the cooking liquid is reduced to 1/4 cup, about 35 minutes.
Transfer the vegetables and their cooking liquid to a food processor and let cool for 10 minutes. Add the remaining 1 tablespoon of butter and the milk and process until smooth. Season the celery rootturnip puree with salt and pepper, transfer to a bowl and serve.