Begin by pounding the top sirloin steak until it's about ¼ inch thick. Slice the beef into 1-inch strips and place in a large mixing bowl.
Sprinkle 2 tablespoons of cornstarch over the beef strips and toss well to coat.
In a separate bowl, whisk together the soy sauce, brown sugar, hoisin sauce, water, red pepper flakes, and black pepper.
Heat the canola oil in a large skillet or wok over medium-high heat.
Add coated beef strips to the skillet in a single layer. Cook for 2-3 minutes on each side until browned and cooked through.
Push the beef to one side of the skillet, add minced garlic to the empty side, and sauté for about 30 seconds until fragrant. Pour sauce mixture over the beef and stir together. Simmer for 2-3 minutes until sauce thickens slightly.
Add green onions to the skillet, tossing them with the beef and sauce. Cook for an additional minute.
Serve hot over steamed rice or noodles.
