Preheat the oven to 350 degrees F (175 degrees C). Lightly grease a 9x13-inch baking dish.

Place bacon in a large skillet and cook over medium-high heat, turning occasionally, until evenly browned, about 10 minutes. Drain bacon slices on paper towels and let cool; crumble.

Place bacon, stuffing, Parmesan cheese, Worcestershire, soy sauce, egg, onion soup mix, salt, and pepper in a large bowl and mix well. Divide meat mixture into 6 individual portions. Mold each portion into oval-like shapes. Press top of each oval in and add Swiss cheese. Mold meat back up over the tops, enclosing cheese.

Mix ketchup, mustard, and brown sugar together in a separate bowl. Cover each loaf with mixture. Place loaves in the prepared baking dish.

Bake in the preheated oven until a toothpick inserted into the centers comes out clean, about 45 minutes.

While loaves bake, saute onion over medium-high heat in a frying pan, 5 to 7 minutes. Top baked loaves with onions and let stand for 5 minutes to cool before serving.