Cook noodles according to package directions. Drain.
Melt butter in 10-inch skillet until sizzling; add mushrooms. Cook, stirring occasionally, until browned, usually 3-4 minutes.
Combine water and gravy mix in 3-quart saucepan. Cook over medium heat 3-5 minutes or until mixture comes to a boil and is thickened. Remove from heat; stir in sour cream and nutmeg.
Add cooked egg noodles, mushrooms, meatballs and parsley; gently stir to coat.
Spoon noodle mixture into 2-quart round casserole coated with no-stick cooking spray. Bake at 350 F for 30-40 minutes or until heated through.
Recipe by Pocket Change Gourmet at https://pocketchangegourmet.com/swedish-meatball-casserole-recipe/