Place the udon in a large bowl and cover them with enough boiling water to thaw frozen udon or warm fresh udon. Drain the noodles and set aside.

Add the pork to a large skillet and bring to medium-high heat. Season with red pepper flakes and kosher salt. Use a spatula to break the pork into large chunks and cook undisturbed until the pork is sizzling and browning on the bottom, about 4-5 minutes. Stir the ginger into the pork and cook for another 4 minutes or until the sausage is well browned. Transfer the pork to a bowl with a slotted spoon to reserve the rendered fat in the skillet.

Add the cabbage and mushrooms with half of the green onion to the same skillet on medium-high. Cook until the cabbage caramelizes and softens with the mushrooms, about 3-4 minutes. Stir in the noodles and toss with the pork. Whisk the soy sauce, rice vinegar, and sesame oil in a small bowl or jar and pour over the noodles. Toss to coat well and cook for 1-2 more minutes for the saucy flavors to absorb into the noodles and pork. Sprinkle with the rest of the green onion and the toasted sesame seeds and enjoy.