Soak the glass noodles in hot water for 15 minutes, then drain and set aside. .
For the meat seasoning
Slice beef (or pork) into thin match sticks (about 1/4-inch thick) against grain and mix with marinade ingredients. Set aside.
For the Japchae sauce
Combine all japchae sauce ingredients in a bowl. Set aside.
To cook Japchae
Heat 1 tablespoon oil in a large skillet or wok over medium high heat. Add onion, carrot, pepper, and a pinch of salt; stir-fry until soft but have a slight crunch. Add sliced shiitake mushrooms and another pinch of salt. Continue to stir-fry until soft. Add spinach at the end and stir-fry until spinach is wilted. Remove the skillet from the heat and transfer the vegetables to a large plate to cool.
Reheat the pan over high heat with the remaining 1 tablespoon oil, add the beef (or pork) and stir-fry until fully cooked. You will see the juice coming out from the meat. Transfer the meat to the large plate with the reserved vegetables, reserving the juice in the pan.
Add the drained glass noodles to the pan with the meat juices. Pour the japchae sauce over the noodles and toss to combine. Let the noodles cook over medium heat until they are soft and the sauce liquid is mostly absorbed into the noodles, about 3-4 minutes.
Reduce the heat to low. Add the vegetables and meat back to the pan over the noodles. Add sesame oil and sesame seeds. Toss well and adjust seasoning to taste.
Video
Notes
Expert Tips:
Let the cooked vegetables and beef cool slightly before combining. Mixing while piping hot can cause excess moisture and soften the noodles.
Use kitchen scissors to trim the noodles into shorter lengths after cooking. This makes serving and eating much easier.
Finish with sesame oil and sesame seeds at the very end. Adding them off heat preserves their aroma.
For gluten-free japchae, use gluten-free soy sauce such as tamari.
For a vegetarian or vegan version, omit the beef and skip the meat marinade. The noodles and vegetables still carry plenty of flavor.
Storage & Reheating:Store in an airtight container in the refrigerator for up to 3 days.Reheat gently in a skillet over low heat with a small splash of water to loosen the noodles. Avoid overheating, as the noodles can lose their chewy texture.