3/4 cup grated swiss cheese, such as Gruyere or Comte (2 1/2 ounces)
Coarse kosher salt
Freshly ground black pepper, to taste
Melt butter in a medium skillet over medium heat. Brown the butter, swirling the pan almost constantly until foam subsides and butter is brown with a rich nutty aroma (watch closely as it can easily burn). As soon as the butter browns, stir in pecans and sage leaves. Cook until pecans are toasty and sage begins to crisp, about 1-2 minutes. Strain through a fine mesh sieve set over a bowl to catch the brown butter. Season the pecan-sage mixture with ¼ teaspoon salt. Reserve pecans and browned butter separately.
In a small pot or a microwaveable measuring cup, gently warm milk and cream together.
Peel potatoes and cut evenly into 2-inch pieces. Place in a large pot with 1 tablespoon salt and cover with cold water. Bring to a boil over medium heat, then reduce heat to low and simmer until very tender, about 15-20 minutes. Drain potatoes, then return to the same (now-empty) pot. Mash thoroughly with a potato masher (or use a potato ricer). Stir over low heat for 1-2 minutes to allow the potatoes to release some steam.
Stir in browned butter, grated cheese, and 1 teaspoon salt. Slowly stir in warmed milk and cream until you have your desired creamy texture.
Spoon into a serving bowl and top generously with brown butter pecans and sage leaves. Season with freshly cracked black pepper.