In a medium bowl, whisk together low-sodium soy sauce, water, and brown sugar until well combined. Heat 1 tablespoon canola oil in a small saucepan over medium heat, then add minced garlic and ginger. Sauté for about 1 minute until fragrant. Pour in the soy mixture and bring to a boil. Reduce heat to simmer for 5 minutes, letting the flavors meld before setting aside.

Thinly slice the beef sirloin against the grain and place in a bowl. Drizzle with sesame oil and sprinkle in the cornstarch, tossing to coat the beef evenly. Let marinate for 10 minutes.

In a large skillet, heat 1 tablespoon canola oil over medium-high heat. Add the marinated beef in a single layer, cooking for about 2 minutes without stirring. Stir-fry for another minute until just cooked through. Remove from skillet and set aside.

In the same skillet, add more oil if necessary, then add broccoli florets. Stir-fry for about 3 minutes. Add a splash of water and cover the skillet for 1 minute to steam the broccoli.

Return the beef to the skillet with broccoli, pour in the prepared sauce, and stir gently. Cook for an additional minute to warm through and let the sauce thicken slightly. Adjust seasoning with kosher salt and sprinkle red pepper flakes if desired, then garnish with sliced green onions and sesame seeds before serving.