Heat the olive oil in a skillet over medium heat. Add the mushrooms, onion, and garlic. Cook until the mushrooms release their liquid and the onion softens, about 6 to 8 minutes. Remove from heat and let cool slightly.
Place the ground beef in a mixing bowl. Add the cooked mushroom mixture, egg, thyme, salt, pepper, and 1/2 cup of the shredded Swiss cheese. Mix with your hands until the ingredients are evenly combined.
Divide the mixture into four equal portions. Shape each portion into a small loaf and place them on a baking sheet lined with parchment paper.
Bake at 375 degrees for 25 minutes. Remove the baking sheet from the oven and sprinkle the remaining 1/2 cup of Swiss cheese over the top of each loaf. Return to the oven for 5 more minutes until the cheese melts.