In a large mixing bowl, combine the finely chopped yellow onion, red bell pepper, minced garlic, grated ginger, chopped mushrooms, shredded cabbage, scallions, and roasted sesame seeds. Mix well.
Pour in the soy sauce, roasted sesame oil, and sugar. Stir until everything is evenly coated. Then, fold in the panko breadcrumbs to help bind the filling.
Take a gyoza or wonton wrapper and place about a tablespoon of the filling in the center. Use your finger to wet the edges of the wrapper with a little water. Fold the wrapper in half to create a half-moon shape and pinch the edges to seal. You can also create pleats for a traditional look.
Heat a tablespoon of canola oil in a non-stick skillet over medium-high heat. Once hot, add the potstickers in a single layer, making sure not to overcrowd the pan. Fry for about 2-3 minutes until the bottoms are golden brown.
After the bottoms are golden, carefully add about 1/4 cup of water to the skillet and cover immediately. Allow them to steam for about 5-7 minutes, or until the water has evaporated and the potstickers are cooked through.
Remove the lid and let the potstickers cook for another minute to re-crisp the bottoms. Serve them hot with the dipping sauce for an unforgettable flavor experience.