1. In a large skillet, cook beef over medium heat until fully cooked.
2. Drain the fat from the skillet; stir in tomatoes, tomato paste, ketchup, brown sugar, mustard & salt. Bring to a boil then reduce heat and simmer uncovered for 5 minutes.
3. Toast buns, place sloppy joe mixture on one side of the bun top with arugula if desired, and serve.