In a small bowl, toss the sliced beef with soy sauce and cornstarch until evenly coated. Set aside to marinate for 10-15 minutes.
In a separate bowl, whisk together soy sauce, oyster sauce, hoisin sauce, brown sugar, rice vinegar, sesame oil, minced garlic, and grated ginger. Set aside.
Cook the noodles according to the package instructions. Drain and set aside.
Heat vegetable oil in a large skillet or wok over medium-high heat. Add the marinated beef and cook for 2-3 minutes until browned and nearly cooked through.
Add the bell pepper, onion, and snap peas, and stir-fry for another 2-3 minutes until the vegetables are tender-crisp.
Add the cooked noodles to the pan along with the sauce. Toss everything together until the noodles are coated and heated through.
Garnish with sesame seeds and chopped green onions before serving.