Bring 2 quarts of water to a rolling boil. While the water heats, break the 1 lb sweet potato starch noodles in half so they’re easier to handle.
Add the broken noodles to the boiling water and cook 5 minutes or according to the package instructions until tender. Stir once or twice so they don’t clump.
Drain the noodles in a colander and rinse thoroughly with cold water to stop the cooking. Drain well and set the noodles aside in the colander.
In a medium bowl, whisk together the 1/4 cup soy sauce, 2 tbsp maple syrup, and 1 tbsp sesame oil to make the marinade. Reserve about 2 tablespoons of this marinade in a small bowl for cooking the vegetables; keep the rest for tossing the noodles later.
Heat a large wok or wide skillet over medium-high heat. Add the reserved marinade to the hot pan and, once hot, add the 1/2 yellow onion (thinly sliced) and the 2 cloves garlic (minced). Sauté 3–4 minutes, stirring, until the onion is softened and slightly golden.
Add the sliced red bell pepper and the 2 julienned carrots to the pan. Stir-fry 3–4 minutes more, until the vegetables are crisp-tender.
Push the vegetables to the side of the pan, add the 8–10 thinly sliced shiitake mushrooms, and drizzle a little of the remaining marinade over them (you may use some of the marinade you saved or a splash from the bowl). Sauté the mushrooms 4–5 minutes, stirring, until they’re golden, fragrant, and tender.
Add your leafy vegetable choice: if using 2 handfuls of spinach, add it now and cook 1–2 minutes until just wilted; if using 1 crown of chopped broccoli, add it with the bell pepper and carrots in step 6 and cook until broccoli is tender-crisp (about 4–6 minutes total).
Add the cooled noodles to the pan with the vegetables and mushrooms. Pour the remaining marinade over everything. Turn the heat to medium-high and toss or stir-fry 2–3 minutes until the noodles are heated through and evenly coated with the sauce.
Transfer to a serving dish and garnish with sesame seeds and sliced green onion. Serve hot.