Place the pork, mushrooms, cabbage, green onions, soy sauce, sesame oil, cornstarch, salt, pepper, ginger and garlic in a large bowl. Stir until thoroughly combined.
Place one potsticker wrapper on a flat surface. Keep the remaining wrappers covered so that they don't dry out.
Place a little under 2 teaspoons of the pork mixture onto the center of the wrapper. Brush the edges of the wrapper with water.
Fold the wrapper over and hold it open in a half moon shape. Fold the top of the wrapper over to make pleats. See step-by-step photos in the post.
Repeat the process with the remaining wrappers and filling.
Heat 1 tablespoon of the oil in a large pan over medium-high heat. Place 6-8 potstickers in the oil with the flat side down. Cook for 2-3 minutes or until bottoms are browned.
Add 1/4 cup of water to the pan then immediately cover the pan to steam the potstickers.
Steam for 3 minutes, then uncover the pan and reduce the heat to medium. Cook for another 3-4 minutes until all of the water evaporates.
Repeat the process another 2 times with the remaining oil and potstickers.