Start by thinly slicing your flank steak or sirloin against the grain to ensure tenderness. Pat the beef dry with paper towels, then toss it in a bowl with cornstarch until evenly coated. Set aside for about 10 minutes.

In a medium bowl, whisk together the low-sodium soy sauce, oyster sauce, cornstarch, brown sugar, beef broth, and sesame oil until smooth and combined. Set the sauce aside.

Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat. Add the broccoli florets and stir-fry for about 3-4 minutes until vibrant and crisp-tender. Transfer to a plate.

In the same skillet, add another tablespoon of oil and increase the heat to high. Add the sliced beef in a single layer, searing for 1-2 minutes per side until browned. Transfer to the plate with broccoli.

Lower the heat and add garlic and ginger to the skillet, sautéing for about 30 seconds. Pour in the prepared sauce, stirring continuously until it thickens. Add the beef and broccoli back to the skillet and toss to coat.

Serve the Beef and Broccoli Stir-Fry immediately over steamed rice or lo mein noodles. Garnish with sesame seeds and sliced green onions.