Cook the sweet potato noodles according to your packaging. Drain, add a small amount of sesame oil (to keep from sticking) and set aside.
On medium to medium-high heat, saute the onions for 2 to 3 minutes, or until the onions become slightly translucent.
Next, add the carrots and bell pepper. Saute them for about 2 to 3 minutes until the veggies have softened.
Then, add the mushrooms and green onions. Saute for about 1 minute.
Toss the sweet potato noodles back into the saute pan and the japchae sauce. Use a tong to ensure all of the ingredients are combined.
Lastly, add the spinach (you can really add as much as you want!), and stir it into the noodles until it wilts. Salt and pepper to taste, and serve immediately.