INGREDIENTS
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3 cups whole milk (water may be substituted)
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1 1/2 cups water
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2 teaspoons kosher salt, plus more as needed
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2 pounds celery root, also known as celeriac (about 3 small)
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12 ounces Yukon Gold potatoes (about 4 medium)
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6 tablespoons unsalted butter (3/4 stick), at room temperature
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Freshly ground white pepper