Ingredients

• 5 pounds medium russet potatoes, peeled and cut into 1-inch chunks
• 12 tablespoons (1 1/2 sticks) unsalted butter, cut into 1/2-inch cubes
• Kosher salt and freshly ground black pepper
• 1 1/2 to 2 cups milk, warm
• 1 cup sour cream
• 2 cups shredded sharp Cheddar
• 1/3 to 1/2 cup prepared horseradish
• 2 tablespoons sliced fresh chives

Instructions

1. Toss the potatoes, butter, 1 tablespoon salt and 1/2 teaspoon pepper together in a 6-quart slow cooker. Add 1 cup of water, and cook, covered, on high until the potatoes are tender, about 4 hours. (They may have browned lightly in spots, but that is OK.)
2. Add the sour cream and 1 1/2 cups of the warm milk to the potatoes, and smash them with a potato masher until they are mostly smooth. If they need to be thinned out, stir in the remaining 1/2 cup warm milk. Add most the Cheddar and all the horseradish, and stir to combine. Season to taste with salt and pepper. Garnish with the remaining Cheddar and the chives.