In a small bowl, whisk together all the sauce ingredients. Set aside.
Bring a medium pot of water to a boil over high heat. Reduce heat and keep it at a simmer.
In a large flat griddle over medium high heat. Add 2 tablespoons vegetable oil and grease the griddle. Cook the pork belly until no longer pink. Reduce heat to medium. Add onion. Cook for 2 minutes. Add carrot and cabbage. Cook until almost tender, about 3 – 4 minutes.
Meanwhile, add noodles in the simmering water. With a pair of chopsticks or tongs, separate the noodles as quick as possible without breaking them. Drain and set aside.
Toss in green onion and mushroom. Cook for another minute. Season with ground black pepper. Add the cooked noodles. Combine the noodles with the vegetables.
Turn the heat down to medium-low, add 2/3 of the yakisoba sauce to the noodles. Mix well. Taste and adjust with more sauce according to your own taste. Transfer to serving plates. Sprinkle with seaweed powder and top with pickled red ginger if using. Serve immediately.