In a small bowl, mix all the sauce ingredients until the sugar dissolves. Set aside.
Bring a large pot of water to a boil. Add the sweet potato noodles and boil according to package directions, about 4-5 minutes, or until tender but still chewy. Drain thoroughly and transfer to a large mixing bowl. If the noodles are very long, cut them a few times with clean kitchen scissors.
Heat 1⁄2 tablespoon of toasted sesame oil in a large nonstick pan over medium-low heat. Add the onions, carrots, red bell peppers, and baby spinach. Stir-fry for about 1-2 minutes, or until the vegetables are slightly softened but still crisp. Transfer to the bowl with the noodles.
Add the remaining 1⁄2 tablespoon of toasted sesame oil to the same pan. Stir-fry the garlic and shiitake mushrooms until the mushrooms are fully cooked and softened.
Add the thinly sliced beef and 2 teaspoons of the prepared sauce. Stir-fry until the beef is fully cooked.
Transfer the beef and mushrooms to the bowl with the noodles and vegetables. Pour in the remaining sauce and toss thoroughly with tongs or, once cool enough to handle, clean, food-safe gloved hands until everything is evenly coated.
Sprinkle with toasted sesame seeds. Serve immediately, warm or at room temperature.