Prep the Beef: Slice the flank steak thinly and set aside.
Make the Sauce: In a small bowl, whisk together soy sauce, brown sugar, rice vinegar, water, cornstarch, garlic, and ginger until smooth.
Cook the Beef: Heat a large pan or skillet over medium-high heat and add vegetable oil. Sear the beef slices in batches for 1-2 minutes per side until browned. Remove and set aside.
Cook the Vegetables: In the same pan, add a splash more oil if needed and stir-fry the vegetables for 3-4 minutes until crisp-tender.
Combine and Finish: Return the beef to the pan and pour in the teriyaki sauce. Stir and cook for 2-3 minutes until the sauce thickens and coats the beef and veggies.
Serve: Serve hot over rice, noodles, or enjoy on its own as a flavorful stir fry!