Uncover, turn up the heat and add in the noodles. These will cook pretty quickly, usually in 4-5 minutes. Towards the end of this cook time, toss in the spinach and green onions. You can totally change out the spinach here for chopped kale or bok choy. You could also choose to add more veggies like matchstick carrots, bean sprouts or corn (among other things). Taste and add more soy sauce or the red pepper flakes if you want to add some heat. Serve immediately as the noodles will continue to soak up stock.