Preheat Oven to 350 Degrees.
In a large bowl, combine the ground beef, egg, diced onion, diced celery (if using), salt, cornflakes (or breadcrumbs) and the 1/4 cup of milk (or beef broth). Mix until combined.
Place the mixture onto a cookie sheet and form it into a loaf shape.
Bake in the preheated oven for 35 minutes.
While the meatloaf bakes, prepare the sauce by mixing together the 1/2 cup tomato sauce (or ketchup), 1/2 cup water, 3 teaspoons brown sugar, and 1 teaspoon mustard.
Remove from the oven, cover the top of the loaf evenly with the sauce, and bake for an additional 15 to 20 minutes.
Remove from the oven and let the meatloaf rest for 5 to 10 minutes before slicing.