Preheat your oven to 375°F (190°C) and lightly grease a 9x13-inch baking dish with nonstick cooking spray.
In a large skillet over medium-high heat, cook the ground beef, breaking it up with a spoon, until it is no longer pink. Stir in the chopped onion, salt, and pepper, and cook for 3-4 more minutes until the onion is softened. Drain off any excess grease.
Spread the uncooked elbow macaroni in an even layer on the bottom of the prepared baking dish. Spoon the cooked beef and onion mixture evenly over the top of the macaroni.
In a medium bowl, whisk together the crushed tomatoes, beef broth, Worcestershire sauce, and yellow mustard. Pour this liquid mixture evenly over the beef and macaroni in the baking dish. Do not stir.
Cover the dish tightly with aluminum foil and bake for 40 minutes. The pasta should be tender and most of the liquid should be absorbed.
Carefully remove the dish from the oven and take off the foil. Stir the casserole well to combine all the ingredients. The mixture will be saucy.
Stir in 1 1/2 cups of the shredded cheddar cheese and all of the cubed Velveeta until the cheeses begin to melt into the hot pasta. Sprinkle the remaining 1/2 cup of cheddar cheese evenly over the top.
Return the casserole to the oven, uncovered, and bake for another 10-15 minutes, or until the cheese on top is completely melted and bubbly. Let it stand for 5-10 minutes before serving to allow it to set slightly.