In a bowl, mix together soy sauce, brown sugar, water, and rice vinegar. Set aside.
In a large ziplock bag, add the sliced flank steak and cornstarch. Seal and shake until the beef is evenly coated. Let rest for 15 minutes.
Heat 1 tablespoon of vegetable oil in a skillet over medium-high heat. Add half of the beef and cook for about 3-4 minutes, browning on all sides. Remove and repeat with the remaining beef.
In the same skillet, add the sesame oil, garlic, and ginger, sautéing for about 30 seconds until fragrant.
Pour in the sauce and bring to a simmer. In a small bowl, mix 1 tablespoon cornstarch with 2 tablespoons water to create a slurry, then stir into the sauce to thicken.
Add the cooked beef back into the skillet, stirring to coat in the sauce. Cook for an additional minute until heated through.
Serve over rice or noodles, garnished with sesame seeds if desired.
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Notes
Store leftovers in an airtight container in the refrigerator for up to 3-4 days. Reheat gently on the stovetop.