1.Whisk sugar, soy sauce, hoisin sauce, vinegar, sesame oil and garlic in a jug. Place beef in a glass bowl. Pour half of sauce mixture over beef; stir to combine. Cover. Refrigerate 30 minutes, to marinate.
2.Meanwhile, place noodles in a large heatproof bowl; cover with boiling water. Stand 2 minutes, until softened. Stir with a fork to separate. Drain.
3.Heat half of peanut oil in a wok on high, swirling to coat. Drain beef from marinade; discard marinade. Stir-fry beef, in batches, 1-2 minutes, until browned. Transfer to a plate.
4.Add remaining peanut oil to wok. Add white onion and carrot; stir-fry 2 minutes. Add capsicum; stir-fry 2 mins. Add snow peas and bok choy; stir-fry 1 minute.
5.Return beef to wok with noodles and remaining sauce mixture. Stir-fry 1-2 minutes, until heated through. Sprinkle over sesame seeds. Top with green onion curls, to serve.
To make green onion curls, slice 1 green onion into long, thin strips. Place in iced water in fridge for 10 mins, until curled. You can also use rump or blade steak for this stir-fry, if preferred.