In a bag, toss to combine beef, garlic, ginger, lemongrass, and oil.
In a bowl, cover noodles with boiling water and leave for 3-4 minutes. Drain and return to bowl. Cook frozen veggies following packet instructions.
Add veggies to noodle bowl and combine. Set aside.
Heat a large non-stick pan or work over medium-high. Add beef and cook for 3-4 minutes, stirring occasionally. Remove beef front pan and set aside for a few minutes to rest.
To the beef pan, add pak choy and spring onions. Cook for 3-4 minutes. Transfer to noodle bowl. Toss and spoon onto serving dish. Sprinkle cashews over and garnish with coriander.