In a medium bowl, whisk together the soy sauce, brown sugar, water, mirin, garlic, and ginger until the sugar is mostly dissolved.

Heat a large skillet over medium-high heat and add the cooking oil.

Pat the beef cubes dry with a paper towel. Add them to the hot skillet in a single layer. Sear for 2 to 3 minutes per side until nicely browned, then remove the beef from the skillet and set aside on a plate.

Pour the soy sauce mixture into the same skillet. Bring to a simmer and cook for 2 minutes, using a wooden spoon to scrape up any browned bits from the bottom.

Whisk the cornstarch and water together, then stir the slurry into the simmering sauce. Cook for 1 more minute until the sauce becomes thick and glossy.

Return the browned beef to the skillet and toss to coat evenly in the sauce. Cook for an additional 1 to 2 minutes until the meat is heated through and cooked to your liking.

Remove from heat and garnish with the sliced green onions and sesame seeds before serving over rice or alongside steamed vegetables.