Korean mandoo dumplings are pretty much like gyoza dumplings, so that's what used here. Homemade stock is important when there are only a few ingredients, unless you really like a particular boxed or canned stock.
Put 2 T. sesame oil in saucepan; turn heat to medium. Add onion; cook, stirring occasionally, until it is softened, about 5 minutes. Add garlic and cook, stirring once or twice, for another minute.
Add stock. Bring almost to a boil, then lower to a simmer. Add plenty of black pepper to taste. Add dumplings and continue to simmer. Cook until dumplings are hot, about 5 - 10 minutes more. Add soy sauce and remaining sesame oil to soup. Garnish with scallion once it's in the individual serving bowl(s).
NOTE: If you are not using homemade, unsalted broth, I would not put in all the soy sauce at once. Put it in one tablespoon at a time and see if the saltiness level is right for you or if it needs more. If you have a salty broth and put in the soy sauce all at once, you could end up ruining it.