In a skillet over medium heat, add the beef.
Break up the ground beef in the skillet as it cooks. (This is very important because you'll want the meat to be cooked into small crumbles)
Reduce the heat to medium-low to maintain a gentle simmer and cook, stirring frequently, until the meat is no longer pink
Add water (if needed), mustard, salt, and pepper. Simmer a few more minutes until the mustard is incorporated
Stir in the onion, cover, and remove from the heat
Spread extra mustard on the bottoms of the buns, before using a slotted spoon to mound the beef over top
Cap with pickles and bun tops