In a large bowl, thoroughly mix 1/4 cup of the tomato soup with the ground beef, rice, egg, onion and salt. Firmly shape into 16 meatballs.
In a small bowl, mix remaining tomato soup with a half soup can of water and the mustard.
Melt the shortening in a large skillet over medium heat. Place garlic and meatballs in the skillet. Cook, gently stirring, until evenly browned; drain. Pour tomato soup and mustard mixture over the meatballs and simmer, stirring occasionally, for 20 minutes, or until meatballs are cooked through.