Begin by soaking the sweet potato glass noodles in warm water for about 30 minutes or until they are soft. 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, sautéing for about 2 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-crisp.
Toss in the sliced shiitake mushrooms and cook for an additional 2 minutes until they soften.
Add the soaked glass noodles to the skillet along with the baby spinach. Pour in the soy sauce, sesame oil, and sugar. Stir well to combine all ingredients and cook for another 3-4 minutes until everything is heated through.
Remove from heat and sprinkle with sesame seeds and chopped green onions before serving.