Whisk together soy sauce, hoisin sauce, sesame oil, sugar, garlic, and red pepper flakes in a bowl. Toss beef with marinade, cover and refrigerate one hour to overnight. Heat peanut oil in a wok or large, nonstick skillet over high heat. Add the green onions, and cook for 5 to 10 seconds before stirring in the beef. Cook and stir untile the beef is no longer pink.