In a bowl, marinate the beef with soy sauce, brown sugar, garlic, black pepper, and sesame oil. Cover and refrigerate for while preparing the other ingredients.
Make the egg garnish by cooking the beaten egg in a pan over low heat. Tilt the egg around to spread it evenly and cook for one minute. Flip it over and let it sit on the pan for 1 more minute. Allow it to cool down and slice it into thin strips.
Boil a pot of water, add the baby spinach and blanch for 1 minute. Drain and transfer in a bowl. Add a teaspoon soy sauce and teaspoon sesame seed oil.
Cook the sweet potato starch noodle according to packaging instruction or until the noodles are soft and chewy. Drain and add it to the bowl with blanched spinach. Add another teaspoon of soy sauce and sesame seed oil. Mix by hand (with gloves) and set aside.
In a large pan or wok, add a little bit of vegetable and stir fry the onion and green onion until soft and translucent. Once done, add it to the bowl.
Next, stir-fry the carrots and red bell pepper for 20-30 seconds. Transfer into the noodle bowl.
Take out the marinated beef from the fridge, cook in the same pan over medium heat for 3-4 minutes until it change color. Combine it with the noodles.
Lastly, add minced garlic, brown sugar, soy sauce, sesame seed oil and ground black pepper. Adjust the measurement according to your own preference.
Mix well and transfer in a serving plate. Top with egg strips and sprinkle sesame seeds.