Finely chop zucchini and cabbage.
In two separate bowls, generously sprinkle salt over the chopped zucchini and cabbage and set aside (for at least 15 minutes) while preparing other ingredients. (This process will draw out water, soften the texture, and add flavor.) Squeeze out as much water as possible from the salted zucchini and cabbage by hand. Transfer to a large mixing bowl.
Prepare all the remaining ingredients and add to the mixing bowl. Mix all ingredients well with your hand.
Heat the pan with 2 tablespoons of vegetable oil over medium high heat. Add the dumplings, making sure they aren’t touching each other. Fry for 1 – 2 minutes, until the bottoms are golden brown. Add 1/3 cup of water to the pan, and cover immediately with a lid. Reduce the heat to medium low, and steam for 4 to 5 minutes. Or cook 2 - 3 minutes on each side over medium heat until golden brown without adding water. If the dumplings are frozen, cook a little longer.
Bring a pot of water to a boil. Add mandu (stirring gently so they don’t stick to the bottom of the pot) a few at a time, and cook until all of them come up to the surface. Continue to cook for another minute or two.
How to freeze dumplings: Place mandu pieces on a tray without pieces touching and freeze for about an hour before storing them in a freezer bag. Frozen mandu don't need to be thawed before being cooked. Just cook a little longer.
This mandu recipe recipe was originally posted in September 2009. It’s been very popular, but I’ve updated it here with more information, new photos and minor changes to the recipe.