In a small bowl combine the cornstarch with 1/4 cup water until the cornstarch is dissolved. Add the soy sauce, brown sugar, ginger, cornstarch, and mirin, if using. Set aside.
Heat 1 tablespoon of the oil in a large skillet over medium-high heat. Brown the steak for about 2 minutes on each side. Remove and add them to the slow cooker. In the same skillet add the remaining tablespoon of oil and turn the heat to medium. Sauté the onions and garlic for about 3 minutes, until slightly softened. Stir the sauce and add it to the pan, and stir to scrape up any browned bits stuck to the bottom of the pan. Pour the sauce over the meat in the slow cooker, and then set the slow cooker to high for 4 hours.
Serve the beef teriyaki over the rice, with the scallions sprinkled on top if desired.