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.