Heat oven to 375°. Fit a baking sheet with a rack and coat lightly with nonstick cooking spray.
In a large bowl, combine ground beef, mushrooms, oats, egg whites, onion salt, oregano and 1/4 tsp of the pepper. Form into a loaf and place on prepared pan. Combine ketchup and brown sugar and spread half on top of meatloaf. Arrange bacon over meatloaf in a crisscross pattern. Bake at 375° for 45 minutes; spread on remaining ketchup mixture and bake for an additional 15 minutes or until internal temperature registers 160°.
Meanwhile, place potatoes in a pot and cover with lightly salted water. Bring to a boil; lower heat and simmer, partially covered, for 15 minutes or until fork-tender. Drain and add broth, olive oil, salt, remaining 1/4 tsp black pepper and the nutmeg. Mash to desired consistency; stir in parsley. Serve with meatloaf.
Using grated mushrooms in place of bread crumbs adds moisture to lean ground beef, chicken or turkey.