In a large skillet, cook beef over medium heat until no longer pink, 5-7 minutes; crumble the meat; drain. Stir in the tomatoes, tomato paste, ketchup, brown sugar, mustard and salt. Bring to a boil. Reduce heat; simmer, uncovered, for 5 minutes. Top meat with arugula. Serve on buns. Freeze option: Freeze cooled meat mixture in freezer containers. To use, partially thaw in refrigerator overnight. Heat through in a saucepan, stirring occasionally and adding a little water if necessary. Serve on buns with arugula if desired.