Bring a pot of water to boil and add the sweet potato glass noodles. Cook according to package instructions (usually around 5-7 minutes), then drain and rinse under cold water.

In a large skillet or wok, heat 1 tablespoon of sesame oil over medium heat. Add sliced onion and carrot, sauté for 2-3 minutes.

Add minced garlic and cook for 30 seconds until fragrant, then add spinach and cook until wilted (about 1-2 minutes).

Add the cooked glass noodles to the skillet with sliced bell pepper and green onions. Stir gently to combine.

In a small bowl, mix soy sauce and sugar, then pour over the noodle mixture. Drizzle with remaining sesame oil and stir-fry for an additional 2-3 minutes.

Transfer the Japchae to a serving dish, sprinkle sesame seeds on top, and serve warm.