Soak the sweet potato glass noodles in warm water for about 30 minutes until softened. Drain and set aside.
In a large skillet or wok, heat the vegetable oil over medium-high heat. Add the sliced onion and minced garlic, and sauté for 2-3 minutes until fragrant.
Add the julienned carrot and sliced bell pepper to the skillet. Stir-fry for another 3-4 minutes until the vegetables are tender.
Add the sliced shiitake mushrooms and baby spinach to the skillet, cooking for an additional 2 minutes until the spinach wilts.
Stir in the soaked glass noodles, soy sauce, sesame oil, and sugar. Toss everything together and cook for another 2-3 minutes until heated through.
Remove from heat and sprinkle with sesame seeds and chopped green onions before serving.