Begin by heating a large skillet over medium-high heat, adding 1 tablespoon of canola oil. Once hot, toss in the yellow onion and sauté for 3 to 5 minutes until soft and translucent.
Next, add the red bell pepper, garlic, ginger, and button mushrooms. Sauté for another 5 to 7 minutes until everything is lovely and tender.
Stir in the shredded cabbage and cook it for another minute, just until it wilts slightly. Remove the skillet from the heat and fold in the finely chopped scallions.
In a small bowl, whisk together the soy sauce, roasted sesame oil, and sugar. Pour this mixture over the vegetable mix, stirring until all is well combined. Finally, add the panko breadcrumbs.
Have a bowl of water nearby. Take a gyoza wrapper, and spoon 1 teaspoon of the filling onto the center. Brush a little water around the edge of the wrapper to help seal it.
Fold the wrapper over, pressing down to secure the edges tightly. Make sure the edges are well sealed and shape the dumpling to have a flat bottom.
When you're ready to cook, heat another tablespoon of canola oil in a large nonstick pan over medium-high heat. When the oil is shimmering, gently place the dumplings in, flat side down, ensuring they don’t touch each other. Fry for about 1 minute until the bottoms are golden brown.
Pour 1/4 cup of water into the pan and immediately cover with a tight-fitting lid. Reduce the heat to medium and let them steam for 3 minutes.
After steaming, remove the lid and let the remaining water evaporate for about 1 minute. Cut one dumpling open to check that the filling is cooked through.
Carefully transfer the potstickers to a plate. Wipe the pan clean with paper towels and repeat the cooking process until all the dumplings are done. Serve them with the dipping sauce you prepared earlier.