Heat a smoker to 250 degrees. In a large bowl, combine the ground beef, breadcrumbs, egg, 1/4 cup milk, diced onion, salt, and black pepper. Mix until just combined, then shape into a loaf.

Place the loaf in the smoker and cook for about 2 hours until it reaches an internal temperature of 160 degrees.

While the meatloaf smokes, stir together the ketchup, brown sugar, and mustard in a small bowl to make the sauce. Set it aside.

Bring a large pot of water to a boil and cook the elbow macaroni according to package directions. Drain and set aside.

In a saucepan, melt the butter over medium heat. Whisk in the flour and cook for one minute. Slowly whisk in the 1 cup milk and continue stirring until the mixture thickens. Add the shredded cheddar cheese and stir until melted and smooth. Fold in the cooked macaroni.

Remove the meatloaf from the smoker and brush the tomato brown sugar sauce over the top. Let it rest for 5 minutes before slicing. Serve slices with the macaroni and cheese on the side.