Cut the beef into small cubed size pieces if not already cut.
Mix the sauce ingredients in a bowl - soy, rice vinegar & fish sauce.
Heat a skillet and add the oil. Fry the beef cubes for a few minutes with the coriander and cumin spice and when they are slightly brown (not cooked and not all brown) remove and set aside.
Same pan, add a little more oil if needed then add the onion and garlic and cook for 5 minutes. It will turn brown quickly because of the flavors left from the beef so scrape up the flavors and don't over brown the onions and garlic.
Return the beef cubes and add the peppers, cook for a minute or 2.
Add the sauce ingredients - soy, rice vinegar and fish sauce.
Add the spinach.
Meanwhile cook the noodles in a separate pot as per packet instructions. Generally depending on the type of noodles they cook quickly in and out the water. (See under Notes)
Using a pair of tongs lift the noodles into the beef pan. Add a little of the noodle water if necessary into the beef stir fry just to make a little sauce (See under Notes)