Cook rice noodles according to package instructions. Drain and set aside.
Whisk the stir-fry sauce in a small bowl and place half in a large bowl, reserving the other half for later.
Place the sirloin strips in the bowl with half of the marinade and toss well.
Heat a wok or large skillet over high heat. Add avocado or canola oil to pan. Heat for 30-45 seconds before adding beef to the pan. Begin stir-frying beef for about 2 minutes to form a bit of a crust. Remove beef from pan and set aside.
To the same pan, add the onions and saute another minute or two until they begin to caramelize. Place the reserved, cooked rice noodles to the pan, along with the remaining stir-fry sauce and reserved beef. (Be sure to include any juices that have collected!) Stir in the scallions right before serving.
Be careful not to overcook the rice noodles. When in doubt, err on the side of undercooked as you don't want to overcook and end up with mushy rice noodles.