Prepare the beef by patting the thin slices of lean beef sirloin dry with paper towels, trimming any visible excess fat, and setting the slices aside on a plate.

In a small bowl, make the sauce by whisking together the low-sodium soy sauce, water, honey, rice vinegar, cornstarch, black pepper, and red pepper flakes (if using) until the cornstarch is fully dissolved and the mixture is smooth; set the bowl aside.

Bring a medium pot of water to a boil, add the fresh broccoli florets, and blanch them for about 1 to 2 minutes until they turn bright green and just begin to soften, then immediately drain and rinse under cold water to stop the cooking; set the drained broccoli aside.

Heat the olive oil in a large nonstick skillet or wok over medium-high heat until it shimmers, then add the sliced beef in a single layer and cook for 2 to 3 minutes, stirring occasionally, until the beef is browned on the outside but still slightly pink in the center.

Add the minced garlic and finely grated fresh ginger to the skillet with the browned beef, and sauté for about 30 seconds, stirring constantly, just until fragrant without letting the garlic burn.

Give the prepared sauce a quick stir, then pour it into the skillet with the beef, stirring well to coat all the slices; cook for 1 to 2 minutes, stirring frequently, until the sauce begins to thicken and gently bubble.

Add the blanched broccoli florets to the skillet, tossing them with the beef and sauce so everything is evenly coated, and cook for another 2 to 3 minutes until the broccoli is crisp-tender and the beef is cooked through.

Drizzle the toasted sesame oil over the beef and broccoli, toss once more to distribute the flavor, then remove the skillet from the heat and let the stir-fry rest for 1 to 2 minutes so the sauce can settle and cling to the ingredients.

Transfer the healthy beef and broccoli to serving plates or a large platter, sprinkle the sliced green onions over the top as a fresh garnish, and serve warm.