Combine soy sauce, sesame oil, minced garlic, grated ginger, and brown sugar to create the marinade. Add the thinly sliced beef sirloin to the marinade, ensuring it is well coated, and let it marinate for at least 15 minutes.

Cook Korean noodles according to package instructions until al dente, then drain and set aside. Slice the onion, julienne the carrots, and slice the bell pepper, preparing all vegetables for cooking.

Heat vegetable oil in a large skillet or wok over medium-high heat. Add the marinated beef to the hot skillet and cook until browned, about 3-5 minutes, then remove the beef and set it aside.

In the same skillet, add the sliced onion, julienned carrots, and sliced bell pepper, cooking until the vegetables are tender-crisp. Return the cooked beef to the skillet with the vegetables.

Add the cooked noodles and fresh spinach to the skillet, tossing everything together until the spinach wilts and all ingredients are evenly combined and heated through. Serve immediately.