In a bowl, combine the thinly sliced flank steak with soy sauce and cornstarch, mixing well until coated evenly. Let it marinate for about 10 minutes while you prepare your vegetables.
While the steak marinates, slice the bell pepper, julienne the carrot, and separate the broccoli into florets. Mince the garlic and ginger, then chop the green onions, setting everything aside in bowls ready for quick access during cooking.
Heat two tablespoons of vegetable oil in a large skillet or wok over medium-high heat until shimmering but not smoking.
Add the marinated flank steak to the hot skillet in a single layer, cooking it without stirring for about 2-3 minutes until browned on one side. Flip the steak pieces and continue cooking until just cooked through; then remove them from the pan and set aside.
In the same pan, add more oil if needed, followed by minced garlic and ginger until fragrant—about 30 seconds. Toss in all prepared vegetables along with shrimp, stirring frequently until shrimp turns pink and veggies are crisp-tender.
Return the cooked steak back into the skillet with vegetables and shrimp, seasoning with salt and pepper to taste. Stir everything together well, letting all flavors meld before serving hot garnished with chopped green onions and sesame seeds on top.