Heat vegetable oil in a large skillet or pot over medium-high heat.
Add the thinly sliced beef sirloin and cook for 3-4 minutes until browned. Remove beef from the pot and set aside.
In the same pot, add the sliced onion and minced garlic. Sauté for 2-3 minutes until the onion becomes translucent.
Add the sliced mushrooms, broccoli florets, and julienned carrot to the pot. Cook for 5 minutes, stirring occasionally, until the vegetables are tender-crisp.
In a small bowl, whisk together soy sauce, brown sugar, rice vinegar, sesame oil, and beef broth.
Return the browned beef to the pot and pour the sauce mixture over the beef and vegetables. Stir to combine.
Bring the mixture to a simmer and cook for 5 minutes to allow flavors to meld.
In a separate small bowl, mix cornstarch and water until smooth. Slowly add the cornstarch slurry to the pot, stirring constantly until the sauce thickens, about 1-2 minutes.
Remove from heat. Serve the beef and vegetable mixture over cooked white rice.
Garnish with toasted sesame seeds and sliced green onions before serving.