Preheat the oven to 350 degrees and lightly grease a loaf pan.
In a large bowl, combine the ground beef, mushrooms, onion, crushed crackers, milk, salt, pepper, garlic powder, and 3/4 cup of the Swiss cheese. Mix just until the ingredients hold together.
Press the mixture evenly into the prepared loaf pan. Bake for 55 to 60 minutes until the internal temperature reaches 160 degrees.
While the meatloaf bakes, bring a large pot of salted water to a boil. Cook the egg noodles according to package directions, then drain.
Return the noodles to the pot and stir in 3 tablespoons of butter until melted and evenly coated.
Remove the meatloaf from the oven and sprinkle the remaining 1/4 cup Swiss cheese over the top. Let it rest for 5 minutes so the cheese melts.
Slice the meatloaf and serve the pieces over the buttered noodles. Finish with a sprinkle of parsley.