Prepare the Sauce: In a small bowl, whisk together the soy sauce, brown sugar, and water or broth until the sugar dissolves. Stir in the sriracha and sesame oil if using, then set the sauce aside so it’s ready to go when you start cooking.

Slice and Coat the Steak: Slice the flank steak or sirloin into thin strips, about ¼ inch thick or slightly thinner for extra tenderness. Sprinkle with cornstarch and toss until evenly coated; this is what gives the beef that silky, restaurant-style texture.

Sear the Beef: Heat 1 to 2 tablespoons of a high smoke point oil, such as grapeseed, peanut, or canola oil, in a large skillet over medium-high heat. Once hot, add the steak in a single layer so it sears properly. Cook for 3 to 5 minutes, stirring occasionally, until browned and cooked through. Transfer the beef to a plate to avoid overcooking.

Cook the Vegetables: In the same skillet, add the green beans and cook for 2 to 3 minutes, stirring frequently, until they are bright green and just tender-crisp. Add the chopped white parts of the green onions and continue cooking for another 1 to 2 minutes. Stir in the garlic and ginger, cooking just until fragrant.

Combine and Thicken the Sauce: Return the steak to the skillet and toss everything together. Turn the heat to high, pour in the prepared sauce, and bring it to a gentle simmer. Cook for about 1 minute, stirring constantly, until the sauce thickens and coats the beef and green beans evenly.

Serve: Serve the beef and green bean stir fry immediately over warm rice, and finish with the reserved green onion tops for a fresh, flavorful touch.