In a small bowl, whisk together 3 tbsp of soy sauce, the oyster sauce, 1 tbsp brown sugar, and the ginger and garlic. Toss with the beef and marinate at least 30 minutes.
In a small saucepan combine mirin, the remaining ¼ cup of soy sauce, and remaining tbsp of brown sugar. Bring to a boil and reduce heat to simmer over low so that it reduces until thickened and very syrupy. Set aside.
Heat a large nonstick skillet over medium-high heat. Add vegetable oil and sear beef about 1-2 minutes each side, turning to brown all sides. Add the onion to the pan and stir until it begins to soften, about a minute.
Add the blanched broccolini and stir. Add a couple of tbsp of the mirin/soy syrup reduction, mix well, and serve.
Optional garnishes: toasted sesame seeds and sliced scallion.