Prepare the rice noodles according to the package instructions. This typically involves soaking them in water for several minutes and/or giving them a quick boil. Try to leave them slightly al dente because they will cook more later on in the pan. Slice the mushrooms, carrots, celery, and scallions.
Add the brown sugar, soy sauce, vegetable broth, orange juice, and cornstarch to a medium-sized bowl. Whisk this together until it’s blended.
Heat a large skillet to medium-high, then add the sesame oil. Give the oil a few seconds to heat up before adding in the mushrooms, carrots, and celery. Stir the vegetables around as they cook for 3-4 minutes. Cook them a bit longer if you’d like them to be softer.
Add the sauce mixture and seasoning to the pan. Stir everything together as it cooks for 2-3 minutes. Make sure to scrape the bottom of the pan as you stir because the sauce will begin to thicken at this point.
Turn the heat to low and add the drained rice noodles to the pan. Toss them in with the sauce and veggies for 1-2 minutes. Remove the pan from the heat. Garnish the noodles with scallions and sesame seeds.