Bring a medium pot of water to boil. Add sweet potato noodles. Boil for 5 to 6 minutes. Drain and toss with 1 tablespoon soy sauce and 1 tablespoon sesame oil and set aside.
In a small bowl, mix remaining 2 tablespoons soy sauce and 2 teaspoon sugar. Set aside.
In a large pan, heat remaining 2 tablespoon sesame oil. Add garlic and onion and fry until just softened, about 1 to 2 minutes. Add carrots and red bell pepper and stir fry for another two minutes.
Add spinach, cooked noodles, ground pepper, and soy sauce mixture. Cook for another 2 to 3 minutes.
Add green onions and toasted sesame seed. Stir to mix well.