Trim bok choy and separate dark green tops from white stems. Leave the smaller tops whole, cut the larger ones in half. Thinly slice the stems about ⅓-inch wide.
Bring a large pot of salted water to a boil. Add noodles and cook according to package directions. The fresh udon took about 4 minutes. Drain and run under cool water; drain again.
Heat 2 teaspoons oil in a large skillet over medium heat. Add pork and cook, breaking up with a wooden spatula, until golden and cooked through, about 10 minutes. Season with ½ teaspoon kosher salt, 1½ tablespoons soy sauce and ½ tablespoon rice vinegar. Use a slotted spoon to transfer meat to a bowl. Wipe the pan of any residual oil from the pork.
Add remaining 1 tablespoon oil to the pan. Stir in half the scallions, finely chopped ginger, garlic, and chile flakes. Cook until fragrant, about 1 minute. Add bok choy stems and cook until almost tender, about 3 minutes. Toss in leaves and bean sprouts. Cook for another minute or two. Return pork to the skillet.
Toss noodles, remaining 2½ tablespoons soy sauce and 1 tablespoon rice vinegar and sesame oil into the pan. Cook until just warmed through. Taste and adjust seasoning.
Transfer to a large wide bowl and toss with remaining scallions. Sprinkle sesame seeds and cilantro over the top, along with thinly sliced peppers.
Recipe by My Lilikoi Kitchen at https://www.mylilikoikitchen.com/2018/04/12/ginger-pork-udon-with-baby-bok-choy/