Heat the vegetable oil in a large skillet or wok over medium-high heat. Add the chicken and cook for 5 minutes or until almost cooked through.
Stir in the sliced green onions (white and light green parts only; reserve the dark ends for garnishing), carrot, red bell pepper, mushrooms and white cabbage and stir fry for 5 minutes.
Add the garlic and red chili flakes and continue to stir fry for 2-3 minutes.
Next, stir in the soy sauce, oyster sauce, mirin and brown sugar.
Add the udon noodles and toss to combine. Cook for 1-2 minutes until the noodles are heated through.
Serve immediately with sesame seeds and sliced green onions on top if you like.
