1Prepare all ingredients before you start cooking as yakisoba comes together very quickly. If using fresh yakisoba noodles from a package, loosen them gently with your hands and set aside - they are usually pre-cooked and just need to be heated through. If using dried ramen noodles, cook according to package directions until just al dente, drain, rinse with cold water to stop cooking, and toss with a little oil to prevent sticking. Slice the pork into thin bite-sized strips - pork belly adds more flavor and richness, while pork loin is leaner. Shred the cabbage into bite-sized pieces, julienne the carrot into thin matchsticks, slice the onion thinly, and cut the green onions into 2-inch pieces, separating the white and green parts.
2Make the yakisoba sauce by combining the yakisoba sauce (if you have it), Worcestershire sauce, oyster sauce, soy sauce, and sugar in a small bowl. Stir until the sugar dissolves. If you do not have bottled yakisoba sauce, you can make a substitute by combining 3 tablespoons Worcestershire sauce, 2 tablespoons ketchup, 1 tablespoon soy sauce, and 1 tablespoon oyster sauce. The sauce should be sweet, tangy, and savory - the distinctive flavor that defines yakisoba. Set the sauce aside within easy reach of your cooking station.
3Heat a large wok, flat griddle, or wide skillet over high heat until very hot. Add 1 tablespoon of the vegetable oil and swirl to coat. Add the sliced pork in a single layer and let it sear without stirring for about 1 minute until browned on the bottom. Stir-fry for another 1-2 minutes until the pork is cooked through and has some caramelized edges. Season lightly with salt and pepper. Transfer the pork to a plate and set aside. Do not wipe out the pan - the pork fat adds flavor.
4Add the remaining tablespoon of oil to the hot pan. Add the white parts of the green onions, sliced onion, and julienned carrot. Stir-fry over high heat for 1-2 minutes until slightly softened but still crisp. Add the shredded cabbage and stir-fry for another 2-3 minutes until the cabbage is wilted but still has some crunch. The vegetables should have some charred edges from the high heat - this adds flavor. Season lightly with salt and pepper. Push the vegetables to the side of the pan.
5Add the noodles to the center of the pan. Let them sear against the hot surface for about 30 seconds without stirring to develop some crispy, caramelized bits - this texture is highly prized in good yakisoba. Toss the noodles with the vegetables and continue stir-frying for 1-2 minutes. Return the cooked pork to the pan. Pour the prepared sauce over everything and toss vigorously to coat the noodles and vegetables evenly. The sauce should sizzle and caramelize slightly on contact with the hot pan. Continue tossing for another minute until everything is well combined and the noodles are heated through and glossy with sauce.
6Add the green parts of the green onions and toss briefly. Taste and adjust seasoning if needed - you may want more sauce, a splash of soy sauce, or a pinch of salt. Transfer the yakisoba to serving plates, mounding it attractively. Top generously with the traditional garnishes: a sprinkle of ao-nori (green seaweed flakes), a pile of beni shoga (red pickled ginger), a handful of katsuobushi (bonito flakes) which will dance and wave from the residual heat, and a drizzle of Japanese mayonnaise in a zigzag pattern. These toppings are not optional - they are essential to the authentic yakisoba experience, each adding its own flavor and texture. Serve immediately while hot. Yakisoba is also delicious stuffed into a hot dog bun to make yakisoba pan, a popular Japanese convenience store snack.