Heat a skillet over medium heat. Add the chopped mushrooms and diced onion. Cook until the mushrooms release their liquid and the onion softens, about 6 minutes. Stir in the garlic and cook 1 minute more. Remove from heat and let cool slightly.

In a large bowl, combine the ground beef, breadcrumbs, egg, Worcestershire sauce, salt, pepper, and 1/2 cup shredded Swiss cheese. Add the cooled mushroom mixture and mix gently with your hands until just combined.

Divide the mixture into 4 equal portions. Shape each into a small loaf and place on a parchment-lined baking sheet. Bake at 375°F for 20 minutes.

Remove the pan from the oven. Top each mini meatloaf with a slice of Swiss cheese and sprinkle the remaining shredded Swiss over the tops. Return to the oven for 5 minutes, until the cheese melts and begins to brown.