01 - Combine beef strips with soy sauce, cornstarch, and sesame oil in a bowl. Mix thoroughly to coat and let marinate for 10 minutes at room temperature.
02 - Cook noodles according to package directions until al dente. Drain well and set aside, tossing with a small amount of oil to prevent sticking if needed.
03 - Whisk together soy sauce, oyster sauce, hoisin sauce, sesame oil, brown sugar, water, and cornstarch in a small bowl until cornstarch is fully dissolved and mixture is smooth.
04 - Heat vegetable oil in a large skillet or wok over medium-high heat until shimmering. Add marinated beef in a single layer and stir-fry for 2-3 minutes until browned on edges but still pink in center. Remove beef from pan and set aside.
05 - Return pan to high heat. Add broccoli, carrot, and bell pepper. Stir-fry for 2-3 minutes until vegetables are crisp-tender. Add garlic and cook for 30 seconds until fragrant.
06 - Return cooked beef to the pan along with noodles and green onions. Pour sauce over the top and toss everything together vigorously for 2 minutes until sauce thickens and coats all components evenly.
07 - Transfer to a serving platter or individual plates. Sprinkle generously with toasted sesame seeds and reserved green onion slices. Serve immediately while hot.