Cut the sweet potato starch noodles in half using a pair of kitchen scissors. Bring a saucepan of water to a rolling boil over heat. Add the noodles and cook according to the packet instructions until soft and chewy. Drain the noodles and rinse under cold running water until chilled. Combine the noodles with 2 tablespoons of the dressing in a large bowl and mix well.
Heat 1 tablespoon of vegetable oil in a large non-stick frying pan or wok over medium heat. Add the noodles and stir-fry for 3-4 minutes, until the noodles are translucent. Return the noodles to the bowl.
In the same pan, heat another tablespoon of oil, add the onion and spring onion and stir-fry for 1-minute, until the onion is a little translucent. Add the carrots and stir-fry for another minute, then transfer to the bowl with the noodles.
Add 1-2 teaspoons of oil to the same pan, add the mushroom and marinated beef and stir-fry for 2-3 minutes, until the beef has browned. Add to the noodles.
Add the seasoned spinach to the noodle bowl, pour over the remaining dressing, and toss everything together with your hands. Taste and adjust the seasoning accordingly. Divide among serving bowls and serve warm.