In a medium bowl, combine the soy sauce, mirin, oyster sauce, sugar, dashi powder, and Worcestershire sauce. Set aside.
Bring a large pot of water to a boil. Once boiling, reduce the heat to a simmer. Maintain a gentle simmer until it is time to add the noodles.
Heat the vegetable oil in a large wok over medium-high heat. Add the pork and sauté until browned, about 8 minutes.
Add the garlic and ginger to the pork and cook for 30 seconds until fragrant, stirring constantly.
Add the prepared sauce, cabbage, shiitake mushrooms, and carrots to the wok and stir fry for 5 minutes or until the vegetables are slightly tender.
Add the udon noodles to the pot of simmering water and cook for 1-2 minutes, gently separating them with a chopstick as they reheat in the water. Drain the noodles immediately, rinse with cold water, and set aside.
Add the green onions to the wok and cook for 2 minutes, stirring occasionally.
Carefully incorporate the noodles into the wok until all the ingredients are well combined.
Garnish with freshly chopped green onions and sesame seeds, if desired.