Make sure all your veggies are chopped before beginning to cook. Things move fast, so this is crucial. Now begin making your base sauce/marinate. This will be used multiple times throughout the cooking process. In a bowl add the soy sauce, brown sugar, sesame oil, rice vinegar, mirin, minced garlic, toasted sesame seeds, and black pepper. Use a whisk to mix all the sauce ingredients together and set aside for later.
Cut the beef into thin strips. Try to cut against the grain for optimal tenderness.
Add the sliced meat into a bowl and mix it with 2 Tbsp of the marinate you prepared earlier. Then set it aside to marinate for at least 5-10 minutes.
Bring a pot of water to a boil and cook the glass noodles for about 6 minutes. Transfer the cooked pasta into a fine-mesh sieve to drain the water out and rinse with cold water to prevent the noodles from becoming over-cooked and mushy. Use kitchen sheers to cut the noodles about 4 times so that the noodles aren’t too long.
Then transfer the glass noodles into a large bowl and mix it with 3 Tbsp of the reserved sauce/marinade. Set aside.
Now bring another pot of water to a boil and blanch the spinach for about 10 seconds.
Drain and squeeze the water out and cut it about 2 times. Set aside.
Now bring a wok over medium-high heat and add 1 Tbsp of olive oil once the wok is well heated. Then add the sliced onion + a pinch of salt and stir fry quickly just to soften it but still leave a little crunch to it. Then transfer the onion to a plate to let it cool and stop the cooking process.
In the same wok, repeat the same steps with the green onion. Add 1 Tbsp oil + a pinch of salt and stir fry lightly. With each vegetable, it is important not to overcook them. So transfer to a plate and let it cook to preserve the texture and vibrant color.
Add another Tbsp of oil + salt along with the thinly julienned carrots and stir fry for about 1 minute, then add in the bell peppers. Stir fry and transfer to cool.
Now add in the marinaded beef strips and stir fry for about 1 minute, then add the sliced mushrooms and cook just until tender.
Transfer to the same plate as the vegetables. Tip: spreading out the vegetables keeps the colors vibrant and the texture crunchy.
Add the cooked vegetables to the bowl of noodles.
Then pour in the remaining sauce.
Mix and enjoy. Make sure to taste it before serving in case you prefer it to be saltier.