Heat the oven to 400ºF. Meanwhile, cook 8 chopped bacon slices in a large skillet over medium heat, stirring occasionally, until crisp, 8 to 10 minutes. Turn off the heat. Using a slotted spoon, transfer the bacon to a small bowl.
Stir 2 ½ pounds peeled and diced Yukon gold potatoes, ¾ cup mayonnaise, 3 ounces of the shredded mild cheddar cheese (about ¾ cup), 4 minced garlic cloves, 1 tablespoon Dijon mustard, ½ teaspoon kosher salt, and ½ teaspoon black pepper together in a large bowl. Stir in ½ cup of the bacon.
Transfer the potato mixture to a 10-inch cast iron or broiler-safe high-sided skillet and spread into an even layer. Bake until the potatoes are tender, about 1 hour.
Remove the skillet from the oven. Turn the oven on to broil. Sprinkle the remaining 3 ounces shredded cheddar cheese (¾ cup) evenly over the top. Return the skillet to the oven and broil until the cheese on top is melted and browned in spots, 2 to 3 minutes.
Let cool for 10 minutes. Garnish with the remaining bacon and 1 tablespoon finely chopped fresh chives.
