Cook udon noodles according to package directions until al dente. Drain and rinse with cold water to prevent sticking. Set aside. Prepare all vegetables and chicken.

In a small bowl, whisk together soy sauce, mirin, oyster sauce, brown sugar, and sesame oil until the sugar is dissolved.

Heat avocado oil in a large wok or skillet over medium-high heat. Add the chicken and cook until browned and cooked through. Add sliced onion and minced garlic, cooking for one more minute until fragrant.

Add the mushrooms, carrots, and bok choy to the skillet. Stir-fry for 3-4 minutes until the vegetables are tender but still crisp.

Add the cooked udon noodles to the skillet. Pour the sauce over everything and toss to combine. Cook for 1-2 minutes until everything is heated through. Garnish with green onions and sesame seeds before serving.