Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine all the meatloaf ingredients: meatloaf mix, diced red onion, diced celery, eggs, soy sauce, ketchup, dried oregano, chopped parsley, garlic powder, black pepper, quick oats, and light brown sugar.
Shape the mixture into a loaf and place it in a baking pan.
In another bowl, mix together all the topping ingredients: ketchup, stone ground mustard, paprika, and pepper. Brush this mixture over the top of the meatloaf.
Bake in the preheated oven until the internal temperature reaches 160°F (70°C), about 75–85 minutes for a loaf.