Cook noodles according to package directions, omitting salt and fat. Drain and rinse under cold water; drain.
Heat a large skillet or wok over high heat. Add canola oil to pan; swirl to coat. Add onions and steak; stir-fry 1 minute. Add mushrooms and garlic; stir-fry 1 minute. Add spinach; stir-fry 1 minute or until greens wilt.
Combine rice vinegar and the next 4 ingredients (through Sriracha) in a small bowl, stirring with a whisk. Add vinegar mixture to steak mixture; cook 30 seconds, stirring constantly. Stir in noodles, sesame oil, and salt; cook for 1 minute or until noodles are thoroughly heated, tossing to combine. Sprinkle with sesame seeds.
I altered this recipe and used chicken. In addition to the spinach I added green beans and shredded carrot. It was pretty spicy with the two teaspoons sriracha, which we like, but if you don't want it too hot, cut down to 1 tsp sriracha.
I thought this was very good! I love rice noodles so I was glad to have a recipe to use them. The changes I made were vegetable oil instead of canola, no mushrooms. I actually quadrupled the recipe for 7 people. We did have leftovers. I only used 1/4 of the Sriracha and probably had a little too much lime juice. I loved it, the kids were split on the rice noodle part. I would make this again.
WAY loved this recipe! Super easy to make in a snap! Only substitution was the Sriracha - we used Sambal Oelek (ground chili paste). Oh we also used Fettuccine instead of rice noodles. With as much liquid as there is I would increase noodles to more than 6 ounces. A real keeper!