Bring a large pot of water to a boil. Once boiling, add noodles and remove from heat. Cook noodles according to package instructions (mine took approximately 3 minutes). Do not overcook! Drain and rinse noodles with cold water to stop cooking. Set aside.
In a small bowl whisk together all the ingredients for the sauce, making sure sugar is completely dissolved. Set aside.
Heat a large wok or Dutch oven over medium high heat. Add oil. Once hot, add the onions to the pan and sauté for 2-3 minutes or until translucent. Add garlic and sauté for an additional minute, mixing frequently. Add the carrots to the onions and mix well. Allow carrots to cook for 4-5 minutes, stirring frequently, before adding the mushrooms and bell pepper. Allow vegetables to cook tender, but still maintain a soft bite.
Add the spinach and cook until just wilted (you may need to do this in batches) before adding the sauce. Toss well to evenly coat vegetables in the sauce.
Remove from heat and gently mix the noodles with the vegetables**. Top with chopped scallions and sesame seeds, if desired.
For added heat mix with a small serving of Gochujang or Sriracha, if desired.
**If you find that the noodles have clumped and stuck together after cooling, simply rinse with warm water, drain and transfer them to the pot with the vegetables