Preheat oven to 350∘F (175∘C). Lightly grease a standard loaf pan (approx. 9x5 inches).
In a large bowl, combine all the Meatloaf Ingredients: ground meat, chopped onion, bread crumbs, beaten egg, 21 cup tomato sauce, black pepper, salt, and 21 tsp Worcestershire sauce.
Mix the ingredients thoroughly but gently using your hands until just combined. Avoid overmixing.
Press the meat mixture evenly into the prepared loaf pan.
Prepare Glaze: In a small bowl, whisk together all the Glaze Ingredients: 21 cup tomato sauce, water, brown sugar, vinegar, 1 tsp Worcestershire sauce, and dry mustard until smooth.
Pour the prepared glaze evenly over the top of the raw meatloaf in the pan.
Bake for 1 hour to 1 hour 15 minutes, or until the meatloaf is cooked through and the internal temperature reaches 160∘F (71∘C) on a meat thermometer.
Let the meatloaf rest in the pan for about 10 minutes before slicing and serving.