While the beef is cooking, make the sauce. In a small saucepan over medium heat, combine soy sauce, hoisin, honey, vinegar, garlic, ginger, and red pepper flakes. Bring to a simmer. Stir in the cornstarch slurry and cook until the sauce thickens, about 1 minute. This is your perfect beef and broccoli sauce—thick, glossy, and bursting with umami.