Boil sweet potato noodles in a large pot of water for 6-7 minutes until chewy. Drain and rinse under cold water to stop cooking. Toss with 1 teaspoon sesame oil to prevent sticking.

Warm a tablespoon of olive oil in a large skillet over medium heat. Add the sliced beef, cooking it for 3-4 minutes until it turns brown. Once browned, remove the beef from the skillet and set it aside for later use.

Heat another tablespoon of olive oil in the skillet over medium-high heat. Add the carrots and onions, stirring frequently. Cook for 2-3 minutes until they begin to soften but still retain a slight crunch. Ensure the vegetables do not become too soft.

Add the bell peppers to the skillet and sauté them over medium heat for 2 minutes. They should become tender but maintain their bright color, enhancing the dish's overall appearance and texture.

In the same skillet, add the spinach and sauté over medium heat for about 1 minute until it wilts. Ensure the spinach maintains its vibrant green hue and fresh scent, indicating it's perfectly cooked. Stir gently to avoid overcooking or losing its color.

Place the beef back into the skillet, then incorporate the minced garlic. Stir continuously for about 30 seconds, allowing the garlic to release its aroma and infuse the beef with its rich scent, enhancing the overall flavor profile of the dish.

In a small bowl, combine the soy sauce, sugar, and the remaining sesame oil. Pour this mixture over the beef and vegetables in the pan. Stir thoroughly to ensure each piece is evenly coated with the sauce, creating a glossy finish.

Add the cooked sweet potato noodles to the skillet over medium heat. Stir thoroughly to combine, ensuring the noodles are evenly coated with soy sauce and sesame oil. Mix well with the beef and vegetables, allowing the flavors to meld together for about 3 minutes.

Sprinkle salt and pepper over the mixture, tasting as you go to achieve the right balance. Adjust the seasoning to enhance the sweet and savory elements, ensuring the flavors are well-rounded and complement the other ingredients in the dish.

Before serving, sprinkle sesame seeds over the dish. These seeds contribute a nutty fragrance and a subtle crunch, which enhances the overall texture and visual appeal of the japchae. This final touch not only complements the flavors but also adds an inviting finish.