Step 1: Prepare the sauce by whisking together soy sauce, oyster sauce, rice vinegar, brown sugar, sesame oil, and cornstarch with water in a small bowl. Set aside.
Step 2: Cook lo mein noodles according to package instructions. Drain, rinse with cold water to prevent sticking, and set aside.
Step 3: Heat 1 tablespoon of vegetable oil in a large wok or skillet over high heat until smoking hot. Add sliced flank steak in a single layer and sear for 1-2 minutes per side until browned. Do not overcrowd the pan; cook in batches if necessary. Remove steak and set aside.
Step 4: Add another 1 tablespoon of vegetable oil to the wok if needed. Add shrimp and stir-fry for 2-3 minutes until pink and cooked through. Remove shrimp and set aside.
Step 5: Add minced garlic and grated ginger to the wok and stir-fry for 30 seconds until fragrant.
Step 6: Add broccoli florets, julienned carrots, and sliced onions to the wok. Stir-fry for 3-4 minutes until vegetables are crisp-tender.
Step 7: Add sliced red and yellow bell peppers and continue to stir-fry for another 1-2 minutes.
Step 8: Return the cooked steak, shrimp, and noodles to the wok.
Step 9: Whisk the prepared sauce once more and pour it evenly over all ingredients in the wok. Toss continuously for 1-2 minutes, allowing the sauce to thicken and coat everything.
Step 10: Garnish with chopped green onions and sesame seeds before serving immediately.