Cook the udon noodles according to the package instructions, then drain and set aside.
Heat vegetable oil in a large pan over medium-high heat.
Add chicken slices and cook until golden brown, about 3-4 minutes.
Toss in the onion, carrots, bell pepper, mushrooms, and garlic. Stir-fry for 3 minutes until slightly softened.
In a small bowl, mix soy sauce, oyster sauce, mirin, sugar, and black pepper.
Add the cooked udon noodles to the pan and pour the sauce over the top.
Stir-fry everything together for another 2-3 minutes until well combined.
Garnish with chopped green onions, sesame seeds, and chili flakes if desired. Serve hot!