Preheat the oven to 375 degrees and lightly grease a loaf pan.
In a large bowl, combine the ground beef, mushrooms, onion, breadcrumbs, egg, milk, Worcestershire sauce, salt, and pepper. Mix just until everything is evenly distributed.
Press the mixture into the prepared loaf pan and smooth the top. Bake for 45 minutes.
Remove the pan from the oven and lay the Swiss cheese slices over the top. Return it to the oven for 5 more minutes until the cheese melts.
While the meatloaf finishes, bring a large pot of salted water to a boil. Cook the egg noodles according to the package directions, then drain them.
Return the noodles to the pot and stir in the butter until it melts and coats the noodles evenly.
Slice the meatloaf and serve the portions over the buttered noodles.