01 - Combine thinly sliced sirloin, cornstarch, 1 tablespoon soy sauce, and sesame oil in a bowl. Mix thoroughly and let rest for 10 minutes.
02 - Whisk together soy sauce, oyster sauce, sriracha, mirin, brown sugar, and rice vinegar in a small bowl. Set aside.
03 - Boil udon noodles following package directions until tender. Drain and set aside.
04 - Heat 1 tablespoon vegetable oil over high heat in a wok or large skillet. Add marinated beef in a single layer and sear for 1 to 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 to 3 minutes until crisp-tender.
06 - Return seared beef to the wok along with cooked noodles and prepared sauce. Toss continuously for 2 to 3 minutes until everything is evenly coated and heated through.
07 - Stir in sliced spring onions. Remove from heat and garnish with toasted sesame seeds if desired. Serve immediately.