Slice all vegetables and prepare your protein if using.
Mix soy sauce, oyster sauce, sugar, and sesame oil in a small bowl.
Heat oil in a large pan or wok over medium-high heat.
Cook protein until fully cooked and lightly browned, then remove and set aside.
Stir-fry onion, carrot, cabbage, mushrooms, and bell pepper for 3–4 minutes until slightly softened.
Add udon noodles and gently separate them in the pan.
Pour the sauce over the noodles and vegetables, then add cooked protein back in.
Toss everything together and cook for 1–2 minutes until well coated and glossy.
Top with green onions and sesame seeds if desired, then serve hot.