1In a bowl, toss sliced steak with soy sauce, cornstarch, and black pepper. Let it marinate for 15 minutes.
2Cook udon noodles according to package instructions. Drain, rinse briefly, and toss with sesame oil. Set aside.
3Heat olive oil in a large skillet or wok over high heat. Add steak in a single layer and sear for 1–2 minutes per side until browned. Remove and set aside.
4In the same pan, add a splash of oil if needed. Stir-fry garlic, onions, and bell peppers for 3–4 minutes until tender-crisp.
5In a small bowl, whisk together all sauce ingredients except the cornstarch slurry. Pour into the pan and bring to a simmer. Add cornstarch slurry and cook until thickened.
6Return steak to the pan. Add noodles and green onions. Toss everything together until well coated and heated through.
7Divide into bowls. Top with sliced green onions, sesame seeds, and extra cracked black pepper. Serve hot.