Preheat the oven to 375 degrees. In a large bowl combine the ground beef, mushrooms, onion, garlic, 3/4 cup of the Swiss cheese, breadcrumbs, egg, salt, and pepper. Mix until evenly blended.
Shape the mixture into a loaf and place it in a loaf pan. Sprinkle the remaining 1/4 cup Swiss cheese over the top.
Bake the meatloaf for 45 to 50 minutes until it reaches 160 degrees in the center.
While the meatloaf bakes, place the potatoes in a medium pot and cover with water. Bring to a boil and cook until tender, about 15 minutes.
Drain the potatoes, then return them to the pot. Add the buttermilk, butter, and a pinch of salt. Mash until smooth and creamy.
Let the meatloaf rest for 5 minutes before slicing. Serve slices with a scoop of the mashed potatoes on the side.