Preparation

• Bring a 6-quart pot of well-salted water to a boil over high heat. Add the potatoes and bring back to a boil. Reduce the heat to maintain a simmer, and cook until the potatoes are tender when pierced with a fork, 15 to 20 minutes. Reserve 2 cups of the potato cooking water, drain, and return the potatoes to the pot. Add the cheese and coarsely mash the potatoes with a potato masher.
• In an 8-inch skillet, melt the butter over medium-high heat until deep golden brown, about 4 minutes. Add the butter, crème fraîche, horseradish, 1/2 cup of the reserved cooking water, 1-1/2 tsp. salt, and 1/2 tsp. pepper to the potatoes, and mash until smooth, adding more of the reserved potato cooking water to reach your desired consistency. Season to taste with salt and pepper. Transfer to a 2-quart, 9×13-inch, or similar broiler-safe serving dish.
• Position a rack 6 inches from the broiler and heat the broiler on high. Broil until golden brown, about 3 minutes. Garnish with the dill and serve.

Make Ahead Tips

You can prepare the potatoes to be broiled up to 1 day ahead. Refrigerate and return to room temperature before broiling.