Preheat the oven to 350 degrees. Coat a loaf pan with olive oil cooking spray.
Prepare the meatloaf mixture by combining the onion, mushrooms, parmesan, and garlic in a food processor. Pulse until finely minced.
Combine the milk, crushed saltine crackers, bread crumbs, and well-beaten egg in a dish. Season with garlic powder, sea salt, and freshly cracked black pepper to taste and mix well.
In a large bowl, mix the onion mixture, milk mixture, and ground beef until well-combined.
Form into a loaf and place in the prepared loaf pan.
Make the sauce by combining the ketchup, brown sugar, dried mustard, and nutmeg in a small bowl until well-mixed.
Brush most of the sauce over the top of the meatloaf, saving extra sauce for serving.
Bake the meatloaf for 60-90 minutes, or until cooked through.
Let the meatloaf rest for 10 minutes before slicing and serving with additional sauce. Enjoy.