Marinate the beef in the soy sauce, sesame oil, sugar and garlic for at least 30 minutes or overnight if you can
Heat a large skillet over medium-high heat. Add the beef and stir-fry until completely browned, 5 to 7 minutes. Add in the zucchini and carrots and cooked until soft
Add the black pepper and green onions and cook until the green onions are slightly limp but still retain their color, 1 to 2 minutes. Sprinkle with sesame seeds if you like and serve immediately