Preheat oven to 375 degrees F (190 degrees C).

Place bacon in a skillet over medium heat, and cook until brown and crisp. Remove bacon from skillet and drain on paper towels.

Return the skillet to the heat and increase the temperature to medium-high. Add the onion and garlic, and stir fry until the onion is soft and the garlic begins to brown. Add the balsamic vinegar, and cook while stirring for one minute. Add the chopped mushrooms, and cook for three additional minutes. Remove pan from heat and set aside.

Finely chop 4 slices of the bacon (reserving the remaining 4). In a medium bowl, combine the chopped bacon, ground beef, bread crumbs, Italian seasoning, Parmesan cheese, mushroom mixture and egg. Mix well using hands. Season with salt and pepper, and form into two patties.

Open the two halves of the hamburger bun on an ungreased cookie sheet. Place one beef patty on each half, and cover each with one slice of tomato, 2 slices of the reserved bacon and one slice of Swiss cheese.

Bake for 25 minutes in the preheated oven, or until meat has cooked through.