Prepare the beef by slicing it thinly against the grain.
In a large skillet, heat the vegetable oil over medium-high heat.
Add the sliced beef to the pan and sear until browned, about 3-4 minutes.
Remove the beef from the pan and set it aside, leaving the juices.
In the same pan, add chopped onions and sauté for 3-4 minutes until translucent.
Add minced garlic and sauté for an additional minute until fragrant.
Stir in soy sauce, brown sugar, and rice vinegar, mixing until combined.
Return the beef to the pan and stir to coat with the sauce.
Cook for an additional 2-3 minutes until the sauce thickens slightly.
Drizzle sesame oil over the dish and mix well before serving.
Serve the caramelized beef over cooked rice, garnishing with chopped green onions.