Bring a pot of water to boil and add noodles. Follow instructions on package for cooking time (usually about 5 minutes). Drain and rinse under cold water. Drain well and transfer to a bowl. (If noodles are extremely long, cut in half or thirds with scissors). Add sesame oil to the noodles and toss. Set aside.
Put all the ingredients for the sauce in a bowl and stir until the sugar has dissolved. Set aside.
In a large pan or wok over high heat, add canola oil, garlic and onions and cook for 2 minutes, until onions are translucent.
Add mushrooms, bell peppers, carrots and cook for 3-4 minutes until vegetables are tender but still yielding a soft crunch.
Add noodles and spinach and toss well. Add sauce and cook for 1 minute. Turn the heat off and transfer to a plate or bowl. Top with scallions and sesame seeds and serve.
This Japchae Recipe will keep refrigerated for up to 3 days.