Whisk the mirin, soy sauce, vinegar, brown sugar, sesame oil, scallions, ¼ cup vegetable oil and two-thirds of the garlic and ginger in a 9 x 13-inch baking dish. Add the pork and turn to coat. Cover and marinate in the refrigerator for at least 2 hours or overnight.
Heat a grill or grill pan to medium high. Remove the pork from the marinade, allowing the excess to drip off. Transfer the marinade to a small saucepan, bring to a boil over high heat and boil to reduce slightly, about 5 minutes. Set aside to cool.
Meanwhile, grill the pork, turning occasionally, until a meat thermometer inserted into the center reaches 145˚, 20 to 30 minutes. Remove to a cutting board and let rest for 10 minutes.
Heat the remaining 1 tablespoon vegetable oil in a large skillet over medium-high heat. Add the remaining garlic and ginger and cook until fragrant, about 30 seconds. Add the broccolini along with ¾ cup water and the salt. Cover and steam until the broccolini is bright green and crisp-tender, 5 to 6 minutes.
Slice the pork and serve with the broccolini and rice. Drizzle the reduced marinade on top.
