In a large skillet, combine beef, water, mustard, and sugar. Simmer over medium heat until the beef is cooked through, about 5 minutes, breaking up clumps of meat with a spoon.
Drain the fat if desired (I don't, and it is closer to the original recipe if you don't, but you can if you want). Stir in onion and season to taste with salt and pepper (I like 1 teaspoon salt and ½ teaspoon pepper).
Serve meat on buns with toppings on the side such as ketchup, mustard, mayonnaise, and pickles.