Mashed Potato Casserole with Smoked Gouda and Bacon

6 bacon strips, preferably thick-sliced applewood smoked
3 scallions, green and white parts, finely chopped
3 pounds baking potatoes, such as russet or Burbank, peeled and cut into 2-inch chunks
4 tablespoons (½ stick) unsalted butter, plus more for the baking dish
¾ cup sour cream
1/3 cup milk
2 cups (8 ounces) shredded smoked Gouda

This casserole was created as an over-the-top potato accompaniment for a simply prepared pork loin or roast chicken.

Cook the bacon in a large skillet over medium heat, turning occasionally, until crisp and brown, about 10 minutes. Transfer to paper towels to drain and cool. Chop the cooked bacon. Combine the bacon and scallions; set aside.
Place the potatoes in a large saucepan and add enough cold salted water to cover. Cover the saucepan with a lid and bring to a boil over high heat. Reduce the heat to medium and cook with the lid ajar until the potatoes are tender, 20 to 25 minutes. Drain well. Return the potatoes to the empty saucepan. Cook over low heat, stirring often, until the potatoes film the bottom of the saucepan, about 2 minutes.
Butter a 13 x 9-inch baking dish. Add the butter, sour cream, and milk to the potatoes. Mash with a potato masher (or beat with an electric hand mixer on low speed) until smooth. Stir in 1½ cups of the Gouda and two-thirds of the bacon mixture; reserve the remaining bacon mixture. Spread in the baking dish. Sprinkle the remaining ½ cup Gouda on top. (The casserole and the bacon mixture can be prepared up to 8 hours ahead, but keep them separately covered and refrigerated. Remove the bacon mixture from the refrigerator 1 hour before serving.)
Preheat the oven to 350°F. Bake until the cheese melts and the edges of the mashed potatoes are bubbling, about 30 minutes (or about 40 minutes if refrigerated). Sprinkle with the reserved bacon mixture and serve hot.