1. To prepare filling, over medium-high heat, add 1 tablespoon vegetable oil to a hot skillet or wok. Once oil is hot, drop in the carrots and garlic and cook for 4 minutes, stirring constantly. Add the celery and cook for another 2 minutes, while stirring constantly. Next toss in the sesame oil, mushrooms, cabbage, ginger, pepper and chili flakes and coat well with oil.
2. Cook this mixture for another 2 minutes, then add in the green onions. Stir in two tablespoons of soy sauce or tamari and taste for saltiness. Add a few more dashes if necessary and allow to evaporate. Taste, and if it needs more ginger or soy sauce, adjust to your liking. Once all the veggies are nicely sauteed and soft, transfer to a bowl and allow to cool for at least 10 minutes.
3. For assembly, take a teaspoon or slightly more depending on the size of your gyoza skins, and place the veggie filling in the center of your dumpling. Avoid over-stuffing the wrappers. Moisten the edges with a little water and fold over the wrapper to form a half moon. Pinch or press down the edges and make sure they are well sealed. To make a fancy crimped edge, gather a part of the edge and folding it over on to itself by making a little "S". Repeat this folding pattern 3-4 times on each dumpling.
4. To cook, in a hot pan over medium-high heat, add 1 tablespoon of vegetable oil. Drop in your dumplings and spread them out evenly across the pan. Cook for 2-3 minutes until lightly browned, then flip over and brown the other side. When both sides are lightly browned, add in 2-3 tablespoons of water and cover with a lid. Turn the heat down to just medium and allow the water to completely evaporate. Once the water evaporates, check for doneness.
5. Gyoza are done when they are entirely translucent. If there are still some opaque parts, add another tablespoon of water, cover and cook for an additional few minutes. Don't leave them unattended. You are almost done! Flip them over if they are getting too brown on one side and turn down the heat just a touch. Depending on your make of wrappers you may have to repeat the steaming process 2-3 times. Your patience will be rewarded. Serve these with sesame-chili soy sauce.
Source of recipe: Recipe inspired by Trader Joe's frozen gyoza.