Begin by thinly slicing the beef short ribs and place them into a mixing bowl. Add soy sauce, sugar, minced garlic, and black pepper, ensuring the beef is well-coated. Let this marinate for 15–20 minutes at room temperature.

While the beef marinates, grab another mixing bowl and combine soy sauce, sugar, minced garlic, sesame oil, and a pinch of salt. Whisk until the sugar dissolves, creating a flavorful sauce.

Bring a pot of water to a boil and add the sweet potato noodles. Cook according to package instructions (around 6–8 minutes) until soft yet chewy. Drain and rinse them under cold water, then place them into the bowl with your sauce.

Heat a large skillet or wok over medium-high heat with vegetable oil. Toss in the sliced onion, white parts of the green onion, and shiitake mushrooms. Sauté for about 4–5 minutes until onions are translucent and mushrooms are tender, then transfer to the bowl with noodles.

In the same pan, add more oil and stir-fry the carrots and capsicum for around 3–4 minutes until slightly softened yet crunchy. Add baby spinach and the green parts of the onions, cooking until the spinach wilts. Transfer to the noodle bowl.

Increase the heat and add the marinated beef, cooking for approximately 90 seconds until just cooked through. Stir often to prevent sticking, then add to the bowl with noodles and vegetables.

Gently toss everything in the large mixing bowl until evenly coated in the sauce. Finish by sprinkling white sesame seeds on top before serving warm.