In season and full of flavor—it's a win-win!
In a large saucepan, cover the celery root, parsnips, potato, bay leaves, and garlic with 1 inch of water. Bring to a boil over medium-high heat. Season the vegetables with salt and cook until tender, about 20 minutes. Drain the vegetables. Discard the bay leaves.
Transfer the vegetables to a food processor. Add the milk and 4 tbsp. butter. Puree until smooth; season with salt and pepper. Transfer to a bowl. Using a spoon, swirl the top.
Meanwhile, in a medium saucepan, melt the remaining 2 tbsp. butter over medium heat. Add the leek; season. Cook, stirring often, until the leek is tender, 3 to 4 minutes. Add the cream and simmer until reduced slightly, about 2 minutes; season.
Spoon the creamed leeks on top of the vegetable puree.
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