01 - Combine sliced sirloin with cornstarch, 1 tablespoon soy sauce, and 1 teaspoon sesame oil in a medium bowl. Mix thoroughly and set aside for 10 minutes.
02 - Whisk together soy sauce, oyster sauce, sriracha, mirin, brown sugar, and rice vinegar in a small bowl. Set aside.
03 - Cook fresh udon noodles according to package instructions, then drain and set aside.
04 - Heat 1 tablespoon vegetable oil in a large wok or skillet over high heat. Add marinated beef in a single layer and sear for 1–2 minutes until browned but not fully cooked. Remove and set aside.
05 - Add remaining 1 tablespoon vegetable oil to the wok. Stir fry garlic, ginger, bell pepper, carrot, and sugar snap peas for 2–3 minutes until crisp-tender.
06 - Return beef to the wok along with cooked udon noodles and pour in the prepared sauce. Toss to coat evenly and cook for 2–3 minutes until heated through.
07 - Stir in spring onions, remove from heat, garnish with sesame seeds if desired, and serve immediately.