- 4 russet (baking) potatoes, scrubbed
- 2 medium celeriac (celery root), peeled
- Kosher salt
- ¾ cup hot milk
- 3 tablespoons unsalted butter
- Freshly ground black pepper
- Cut the potatoes in half and the celeriac into quarters. Place in a large pot and fill with cold water so that the potatoes and celeriac are covered by a few inches. Add 1 teaspoon salt and place a smaller pot lid on the vegetables to keep them submerged. Bring to a boil. Cook until tender (vegetables should be easily pierced with a paring knife), 15 to 20 minutes. Drain and return the vegetables to the hot pot to remove excess moisture.
- Put the potatoes and celeriac through a food mill or ricer, or mash them well using a fine potato masher. (If mashing, slide off and discard the potato skins.) Combine the vegetables with the milk and stir in the butter. Season with 1 teaspoon salt, or more to taste, and pepper. Serve immediately.