01 - Combine flank steak slices with 1 tbsp soy sauce, cornstarch, and 1 tsp sesame oil in a bowl; let marinate for 10 minutes.
02 - Cook lo mein noodles according to package directions, drain, rinse with cold water, and set aside.
03 - Whisk together soy sauce, oyster sauce, hoisin sauce, rice vinegar, sesame oil, sugar, and black pepper in a small bowl.
04 - Heat 1 tbsp oil in a large wok or skillet over high heat; stir-fry beef for 2 to 3 minutes until browned, then remove and set aside.
05 - Add another 1 tbsp oil to the same pan; stir-fry broccoli and bell pepper for 2 to 3 minutes until tender-crisp. Add garlic and ginger, stir-frying for 30 seconds until aromatic.
06 - Return beef to the pan, add cooked noodles and pour in sauce. Toss for 2 to 3 minutes until evenly coated and heated through.
07 - Stir in sliced green onions, remove from heat, sprinkle with toasted sesame seeds, and serve immediately.