In a large bowl, toss 1 pound flank steak, with ¼ cup cornstarch until the beef is completely coated. Place the bowl in the fridge for 10 minutes while you prepare the sauce and heat the oil.
In a small bowl combine ½ cup soy sauce, ½ cup mirin, ¼ cup lightly packed brown sugar, ½ teaspoon sesame oil, and 2 dashes fish sauce. Set aside.
Heat 2 tablespoons of the 3 tablespoons oil in a 12-inch skillet over medium-high heat.
When the oil is hot, add the chilled beef to the pan. Cook until browned on both sides, about 3-4 minutes. Remove from the pan and set aside.
Add the remaining tablespoon of oil to the same pan. Add 1 teaspoon minced garlic and ½ teaspoon fresh minced ginger and cook for 1-2 minutes.
Pour the sauce into the pan and cook over medium heat for 2-3 minutes.
In a small bowl, mix together 1 teaspoon cornstarch and 1 tablespoon water. Slowly pour it into the sauce, whisking constantly. Continue to cook until the sauerkraut thickens to your liking.
Add the beef back to the pan and toss to coat it in the sauce. Serve immediately. Garnish with green onions and sesame seeds, if desired.