1. Heat a large skillet over high heat. Add oil to pan; swirl to coat. Add 1/2 cup onions, garlic, ginger, and mushrooms; stir-fry 3 minutes. Remove from pan; cool slightly. Combine mushroom mixture, 1 tablespoon soy sauce, hoisin sauce, pepper, and pork in a medium bowl.
2. Arrange 8 gyoza skins on a clean work surface; cover remaining skins with a damp towel to keep them from drying. Spoon about 1 1/2 teaspoons pork mixture in the center of each skin. Moisten edges of skin with water. Fold in half; press edges together with fingertips to seal. Place on a baking sheet sprinkled with cornstarch; cover to prevent drying. Repeat procedure with remaining gyoza skins and pork mixture.
3. Combine 1/4 cup hot water and brown sugar in a small bowl, stirring until sugar dissolves. Add remaining 1/4 cup green onions, remaining 1/4 cup soy sauce, vinegar, and sambal, stirring with a whisk until well combined.
4. Heat a large heavy skillet over high heat. Generously coat pan with cooking spray. Add 10 pot stickers to pan; cook 30 seconds or until browned on one side. Turn pot stickers over; carefully add 1/3 cup water to pan. Cover tightly; steam 4 minutes. Repeat procedure in batches with remaining pot stickers and more water, or follow freezing instructions. After cooking, serve pot stickers immediately with dipping sauce.
TO FREEZE: Freeze dumplings flat on a baking sheet sprinkled with cornstarch 10 minutes or until firm. Place in a large zip-top plastic freezer bag with 1 teaspoon cornstarch; toss. Freeze sauce in a small zip-top plastic freezer bag. Freeze up to 2 months.
TO THAW: Thaw sauce in the microwave at HIGH in 30-second increments. No need to thaw pot stickers.
TO REHEAT: Follow recipe instructions for cooking, placing frozen dumplings in pan and increasing steaming time by 2 minutes.