Heat a large skillet skillet to medium high heat and add a table spoon of oil. Saute the onion for 3-5 minutes until soft. Add the red bell pepper, garlic, ginger and mushrooms and saute for 5-7 minutes or until the veggies have softened. Add the cabbage and let cook for another minutes. Remove from heat and stir in the green onions.
In a small bowl mix the soy sauce, roasted sesame oil, and sugar together and add to the veggies. Stir until well combined and then add the panko bread crumbs.
Have a bowl of water close by and spoon 1 teaspoon of the filling onto dumpling skin. Brush a bit of the water all around the edge of the dumpling skin. Fold over and press to secure edges. Make sure edges are sealed tightly. Shape the dumpling so that it has a flat bottom. Cover loosely with plastic wrap so that it doesn’t dry out.
When you are ready to cook, heat a large nonstick pan with 1 tablespoon of cooking oil over medium-high heat. When the oil is hot, but not smoking, add the dumplings, flat side down, not touching, to the pan. Let fry for 1 minute until the bottoms are light golden brown. Pour 1/4 cup of water into the pan and immediately cover with a tight fitting lid. Turn heat to medium and let the dumplings steam for 3 minutes. Open lid and let the remaining liquid cook off, about 1 minute. Cut into a dumpling to make sure that the filling is cooked through. Remove to plate, wipe the pan clean with paper towels (or wash) and repeat with remaining dumplings. Serve with dipping sauce.
For the Sauce: In a small bowl, stir together all dipping sauce ingredients.