Cook the sauce: In a 14-inch skillet or deep saucepan (a standard 12-inch skillet will be very full at the end), sauté the ground beef and onions until the beef is no longer pink. Drain if necessary. Stir in the tomatoes with their juices, spaghetti sauce, mushrooms, and seasonings. Bring to a boil, and then reduce the heat and maintain a gentle simmer for 20 minutes, uncovered. (Tip: You want the wateriness of the diced tomato juice to concentrate, but to maintain a good level of sauciness in the final product, you don’t want the sauce to reduce and thicken too much.)
Cook the pasta: Meanwhile, cook the broken pasta in salted water according to package directions. Drain well.
Assemble the casserole: Remove the sauce from the heat and stir in the spaghetti. Place half of the mixture in a 9×13 baking dish (I don’t grease), and then sprinkle with half of the mozzarella. Repeat with the remaining spaghetti mixture and cheese. Sprinkle the Parmesan on top.
Make-ahead tip: At this point, the casserole may be cooled, covered, and refrigerated for several days or frozen for several months.
Bake: Preheat the oven to 375℉ and bake for 20-30 minutes, or until hot throughout and lightly golden on top. If the casserole has been refrigerated, let it sit at room temperature while the oven is preheating and cook for a few extra minutes, as needed. If frozen, thaw completely before baking.
Serve: Garnish with optional fresh parsley and serve.
Storage: Refrigerate leftovers, covered or in an airtight container, for up to 5 days.