Slice the flank steak or sirloin thinly against the grain into bite-sized pieces. In a medium bowl, combine the sliced beef with 2 tbsp soy sauce, cornstarch, sesame oil, and black pepper. Mix well to coat and let marinate for at least 15 minutes at room temperature, or up to 30 minutes in the refrigerator.

While the beef marinates, prepare the sauce and onions. In a small bowl, whisk together the beef broth, 1/4 cup soy sauce, mirin, brown sugar, minced garlic, and grated ginger. Set aside.

Heat 1 tablespoon of vegetable oil in a large skillet or Dutch oven over medium-high heat. Add the thinly sliced onions and cook, stirring occasionally, for 8-10 minutes until softened and nicely caramelized. Remove the onions from the skillet and set aside.

Add the remaining 1 tablespoon of vegetable oil to the same skillet. Increase heat to high. Add the marinated beef in a single layer, making sure not to overcrowd the pan. Cook for 1-2 minutes per side until nicely browned. You may need to do this in batches. Remove the browned beef from the skillet and set aside with the onions.

Reduce the heat to medium. Add the rinsed long-grain white rice to the skillet and stir for 1 minute to lightly toast. Pour in the prepared sauce mixture, scraping up any browned bits from the bottom of the pan. Bring to a boil.

Once boiling, return the cooked beef and caramelized onions to the skillet, distributing them evenly over the rice. Reduce the heat to low, cover tightly with a lid, and simmer for 15-18 minutes, or until all the liquid has been absorbed and the rice is tender. Do not lift the lid during this time.

Once the cooking time is complete, remove the skillet from the heat and let it rest, still covered, for 5-10 minutes. This allows the rice to steam further and become fluffier.

Fluff the onion beef rice gently with a fork. Divide among serving bowls and garnish generously with chopped green onions. Serve immediately.