Ensure meat and all vegetables are cut prior to cooking.
Heat vegetable oil in a large wok or skillet over high heat. When oil is near it's smoking point, cook and stir beef until browned, 3 to 4 minutes. It will continue cooking later, so it doesn't need to be fully cooked at this point. You just want a nice sear on it.
Transfer beef to a plate. Add broccoli, bell pepper, carrots, zucchini, and green onion to the center of the wok. Cook and stir vegetables for 2 minutes. Add garlic, ginger, and red pepper flakes and stir well.
Add beef into vegetables and season with soy sauce, fish sauce, and sesame seeds. Continue to cook and stir until vegetables are tender, about 2 more minutes.
Serve hot over rice or noodles.
Video
Notes
Slicing meat that is partially frozen or very cold is easiest. Slice against the grain to get the most tender bite.Cooking tip: Throughout entire recipe, do not overcrowd your wok. You're better off cooking in small batches, setting aside, and combining everything at the end. You want everything to have a nice sear on it but not be soggy and overly cooked.