Ensure meat and all vegetables are cut prior to cooking.
Heat 2 tablespoons vegetable oil in a large wok or skillet over high heat. When oil is near it's smoking point, cook and stir 1 pound beef sirloin 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 2 cups broccoli, 1 red bell pepper, 2 large carrots, 1 small zucchini, and 2 green onions to the center of the wok. Cook and stir vegetables for 2 minutes. Add 1 teaspoon garlic, 1/2 teaspoon ginger, and 1/2 teaspoon red pepper flakes and stir well.
Add beef into vegetables and season with 2 tablespoons soy sauce, 2 tablespoons fish sauce, and 2 tablespoons sesame seeds. Continue to cook and stir until vegetables are tender, about 2 more minutes.
Serve hot over rice or noodles.
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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.