Mix together the orange juice, maple syrup, soy sauce and Sriracha. Set aside.
In a wok, heat the oil. Add the onions, carrots and green beans and cook, stirring all the time, until the vegetables are slightly tender but still have a bite. Add the mushrooms and stir-fry for a minute.
Add the ginger and garlic, stir-fry for a few seconds, then add the cooked noodles.
Stir-fry for a few minutes, stirring frequently with tongs or a ladle. Add the orange-soy sauce mixture and sausage, if using. Mix well, turn off he heat, and serve hot.