Heat the oven to 375°F. Lightly grease 8 cups of a standard muffin tin.

Warm the olive oil in a skillet over medium heat. Add the chopped mushrooms and cook for 5 to 7 minutes, stirring occasionally, until their moisture has evaporated and they are tender. Set them aside to cool slightly.

Add the ground beef, egg, breadcrumbs, 1/4 cup milk, Worcestershire sauce, Dijon mustard, salt, and black pepper to a bowl. Add the cooked mushrooms and mix gently until combined.

Divide the mixture among the 8 prepared muffin cups. Press one cube of Swiss into the center of each portion and cover it with the meat mixture. Bake for 20 to 25 minutes, until the meatloaves are cooked through.

While the meatloaves bake, place the potatoes in a saucepan and cover them with water. Bring to a boil, then cook until the potatoes are fork-tender, about 15 minutes. Drain well.

Melt the butter in a small skillet over medium heat. Swirl the pan as it cooks, and remove it from the heat when the butter smells nutty and has golden-brown flecks. Watch closely so it does not burn.

Mash the drained potatoes with the browned butter, warm milk, and 1/2 teaspoon salt until smooth and fluffy. Spoon or pipe the potatoes over the baked meatloaf muffins, then sprinkle with chopped chives.