Preheat the oven to 375°F and lightly grease a muffin tin. In a large bowl, combine the ground beef, chopped mushrooms, 3/4 cup of the Swiss cheese, breadcrumbs, egg, onion, 2 cloves of minced garlic, thyme, salt, and pepper. Mix until just combined.

Divide the mixture evenly among 8 muffin cups and press down gently. Bake for 25 minutes until the internal temperature reaches 160°F. Sprinkle the remaining 1/4 cup Swiss cheese over the tops during the last 5 minutes of baking.

While the meatloaves bake, place the potatoes in a pot and cover with cold water. Add a pinch of salt and bring to a boil. Cook until the potatoes are fork-tender, about 15 minutes.

Drain the potatoes and return them to the pot. Add the remaining 2 cloves of minced garlic, 3 tablespoons of butter, milk, salt, and pepper. Mash until smooth and creamy.

Remove the meatloaves from the oven and let them rest for 5 minutes. Spoon the garlic mashed potatoes onto plates and top each serving with two mini meatloaves.