Prepare the Noodles

1. Cook the Noodles: Begin by boiling a pot of water. Add the sweet potato noodles (dangmyeon) to the pot and cook according to the package instructions, usually about 7-10 minutes. These noodles should be chewy but not too soft. Drain them, rinse with cold water to stop the cooking process, and set aside. You can drizzle a little sesame oil over them to keep them from sticking.
2. Season the Noodles: Once the noodles are cooked and drained, drizzle them with 1 tablespoon of sesame oil to keep them separated and add flavor. Set aside.

Prepare the Vegetables and Protein

1. Stir-Fry the Vegetables: In a large skillet or wok, heat 1 tablespoon of sesame oil over medium-high heat. Add the onion, carrots, bell peppers, and mushrooms. Stir-fry for 3-5 minutes until the vegetables soften but remain crisp. You want them to retain their color and texture. Add the spinach at the very end, allowing it to wilt quickly. Remove the vegetables from the skillet and set them aside.
2. Cook the Protein: If you’re using beef (or your chosen protein), add a little more sesame oil to the skillet. Stir-fry the beef strips for 2-3 minutes until just cooked through. You can also add garlic at this stage for extra flavor. Once the beef is cooked, remove it from the skillet and set it aside.

Assemble the Japchae

1. Combine Everything: In the same skillet, add the cooked noodles, stir-fried vegetables, and cooked beef (or your protein). Toss everything together gently to mix.
2. Add Seasoning: In a small bowl, mix together the soy sauce, sugar, sesame oil, minced garlic, rice vinegar, and mirin (if using). Pour this sauce over the noodles and toss everything to coat evenly. Continue to stir-fry for another 2-3 minutes, allowing the flavors to meld and the noodles to soak up the sauce.
3. Finishing Touches: Once the dish is well combined and heated through, sprinkle toasted sesame seeds on top and season with ground black pepper to taste.