Slice flank steak against the grain into thin strips. Rinse shrimp under cold water if necessary. Chop bell peppers and cut broccoli into bite-sized florets.
In a bowl, mix soy sauce, minced garlic, and cornstarch. Add flank steak strips and toss to coat evenly; let marinate while preparing other ingredients.
Heat vegetable oil in a large skillet over medium-high heat. Add marinated steak in a single layer and cook for about 3 minutes until browned on one side; flip to cook the other side.
When the steak is almost done, add shrimp and cook for about 2 minutes until they turn pink.
Toss in bell peppers and broccoli; sauté for another 3-4 minutes until veggies are tender-crisp.
Drizzle sesame oil over the stir-fry while stirring to combine flavors; cook for an additional minute before serving.
