Soak the glass noodles in hot water for 15 minutes, then drain and set aside. Slice the beef into thin matchsticks against the grain and mix with the meat marinade ingredients (1 tbsp soy sauce, 1 tsp sugar, 1 tbsp mirim).

Combine all japchae sauce ingredients (4 tbsp soy sauce, water, sugar, mirim, oil, garlic, and pepper) in a bowl and whisk together.

Heat 1 tablespoon of oil in a large skillet over medium-high heat. Add the onion, carrot, bell pepper, and a pinch of salt. Stir-fry until soft with a slight crunch. Add the mushrooms and continue cooking. Wilt the spinach at the end. Transfer all vegetables to a plate.

Reheat the pan with the remaining 1 tablespoon of oil over high heat. Add the marinated beef and stir-fry until fully cooked. Transfer to the plate with the vegetables, reserving the meat juices in the pan.

Add the drained noodles to the pan with the reserved meat juices. Pour the japchae sauce over the noodles and toss to coat. Cook for 3-4 minutes until the noodles are soft and the sauce is absorbed.

Reduce heat to low. Add the vegetables and meat back to the pan along with the sesame oil and sesame seeds. Toss everything together well, adjust seasoning with salt, and serve warm or at room temperature.