Slice the steak into thin strips against the grain and place them in a medium mixing bowl. Season evenly with the kosher salt and black pepper, tossing so the beef is coated on all sides. Add the cornstarch and toss again until each piece is lightly coated. This coating helps keep the beef tender and allows the sauce to cling later.
1 lb (454 g) sirloin steak, 1 tsp (5 g) kosher salt, ½ tsp (1 g) black pepper, ¼ cup (32 g) cornstarch
In a small mixing bowl, combine the water, low-sodium soy sauce, rice vinegar, brown sugar, toasted sesame oil, grated ginger, and minced garlic. Whisk until the mixture looks smooth and the sugar begins to dissolve. Set aside.
¼ cup (60 ml) water, ⅓ cup (80 ml) low-sodium soy sauce, 1 tbsp (15 ml) rice vinegar, ⅓ cup (70 g) packed dark brown sugar, 1 tsp (5 ml) toasted sesame oil, 1 tbsp (15 g) fresh ginger , 2 cloves garlic
Slice the green onions, keeping the white parts separate from the green tops. This allows the whites to soften in the pan while the greens stay fresh for finishing.
4-6 green onion stalks
Heat a large skillet or wok over medium-high heat. Add the neutral oil and let it heat until shimmering. Add the coated beef in an even layer, working in batches if needed to avoid crowding. Cook, stirring occasionally, just until the beef is no longer pink. Transfer the beef to a clean plate.
¼ cup (60 ml) neutral oil
Carefully spoon off excess oil from the pan, leaving about 2 tablespoons (30 ml) behind. Add the white parts of the green onions and cook, stirring, until they begin to soften, about 30 seconds to 1 minute.
Whisk the sauce again, then pour it into the pan. Cook, stirring frequently, until the sauce comes to a simmer and begins to thicken slightly.
Return the beef to the pan and stir until the meat is evenly coated in the sauce and heated through. Turn off the heat. Sprinkle the green onion tops over the beef and garnish with sesame seeds, if using. Serve immediately, over cooked rice if desired.
1 tsp (3 g) toasted sesame seeds, cooked rice