Preheat the oven to 350°F and lightly grease a loaf pan.
In a large bowl, combine the ground beef, chopped mushrooms, shredded Swiss cheese, breadcrumbs, egg, diced onion, salt, and pepper. Mix until evenly blended.
Transfer the mixture to the prepared loaf pan and shape it into a firm loaf.
In a small bowl, stir together the ketchup, brown sugar, and Worcestershire sauce until smooth.
Spread half the glaze over the top of the meatloaf.
Bake for 45 minutes, then remove from the oven and spread the remaining glaze on top.
Return to the oven and bake for another 15 minutes, or until the internal temperature reaches 160°F.
Let the meatloaf rest for 10 minutes before slicing.