Whisk together soy sauce, hoisin sauce, sesame oil, sugar, garlic, and red pepper flakes in a large bowl. Toss beef with marinade; cover the bowl and refrigerate for 1 hour to overnight.

Heat peanut oil in a wok or large, nonstick skillet over high heat. Cook and stir green onions in hot oil for 5 to 10 seconds. Remove beef from marinade and add to green onions in the wok; discard marinade. Cook and stir until beef is no longer pink and begins to brown, about 5 minutes.