In a large mixing bowl, combine the thinly sliced sirloin steak with cornstarch, low sodium soy sauce, and rice vinegar. Toss until well coated.
In your wok or large skillet, heat 1 tablespoon of oil over medium heat. Add the broccoli florets and stir-fry for about 2-3 minutes. Pour in water, cover, and steam for another 3-4 minutes until bright green and tender-crisp. Remove the broccoli and set aside.
In the same pan, add the remaining tablespoon of oil and increase the heat to high. Add the beef in a single layer and let it sear for about 2 minutes without stirring. Stir and cook for another 2-3 minutes until cooked through and browned. Remove and set aside with the broccoli.
In a small bowl, whisk together beef broth, cornstarch, low sodium soy sauce, brown sugar, and minced garlic. Pour into the hot pan and bring to a simmer. Cook for about 2-3 minutes until thickened.
Return the cooked beef and broccoli to the pan, tossing everything together to coat in the sauce. Cook for another minute for flavors to meld.
Serve hot over steamed rice or noodles, garnished with sliced green onions and sesame seeds, if desired.