In a pan over medium-high heat, brown the beef. Break the meat into small chunks with a spatula and cook until no longer pink, about 5 minutes. Drain any excess drippings, but keep the beef in the pan.
Add the salt and pepper to the beef and stir to combine.
Divide the lettuce, tomatoes and onion between bowls and top with cooked beef and the Dijon mustard. Enjoy!