In a bowl, combine the beef mince, mustard, egg white, herbs, salt and pepper and knead well. Shape mixture into meatballs.
Cook the meatballs in a saucepan on low heat so their own fat will grease the saucepan. Add the onion, garlic, more salt and pepper and stir until meatballs are browned. Add tomatoes and simmer 12-15 minutes.
Meanwhile, cook spaghetti according to packet directions. Drain and serve with meatball sauce.