Heat oil in a deep pan or wok over medium heat and cook onions, ginger, and garlic until onions are translucent.
Add mushrooms and bell peppers to the pan. Cook until mushrooms are soft.
Add cabbage, carrots, salt, and pepper. Cook for another 3-4 minutes and remove from heat. Set aside to cool completely.
Once mixture is cool, add cilantro, green onions, soy sauce, sesame oil, cooking sherry, and sugar. Mix well.
Place a spoonful of the mixture onto a wonton wrapper. Dip your finger in water and run it around the edge of the dough.
Fold in half and pinch together, pleating the dough as you go.
Heat more oil in a large skillet and place dumplings in the oil. Cook for 3-4 minutes or until a crust has started to form on the bottom. Pour the water in the pan and cover with a lid. Steam for 6-8 minutes, remove the lid and remove from the pan.