Bring a large pot of salted water to a rapid boil. Cook the spaghetti until just shy of al dente, 8 to 12 minutes, then drain. You want the noodles to be a little under cooked.
While the spaghetti cooks, whisk together all the sauce ingredients in a bowl.
Heat the oil in a large skillet or wok over medium-high heat. Add the mushrooms, bell pepper, carrots, and garlic and cook, stirring frequently, until tender, 3 to 4 minutes. Add the broccoli and snow peas and toss to combine. Season the vegetables with the salt and cook for another 2 to 3 minutes.
Add the noodles and sauce to the skillet and toss to combine with the vegetable mixture. Taste and add more salt, if desired.
Notes
Serve Immediately: Divide the lo mein evenly into 4 bowls. Garnish with green onions and serve warm.Meal Prep: Divide the lo mein evenly into 4 meal prep containers and garnish with green onions. When ready to serve, follow the reheat instructions.Store: In the refrigerator for up to 5 days.Reheat: Heat the lo mein in the microwave 1 to 2 minutes or on the stove top until warm.