Tip the beef strips into a bowl and spoon 1 tbsp Dijon mustard and half of the tamari over the top. Mix well, till the beef is coated on all sides. Set aside to marinate for 20 mins.
While the beef marinates, halve the red pepper. Scoop out the seeds and white bits. Finely slice it. Trim the ends off the spring onions. Finely slice them. Thinly slice the mushrooms. Put a large pan of water on to boil.
Heat up a wok or deep frying pan over a high heat for 2 mins. Add 1 tbsp oil and the marinated beef strips. Stir fry for 3-4 mins till brown on all sides. Lift out of the pan and set aside on a plate.
The water should be boiling in the pan. Add the noodles. Simmer for 5 minutes, then drain.
While the noodles cook, add the sliced veg to the wok. Stir fry for 2-3 mins till just starting to soften.
When the noodles are cooked, drain them well. Add to the wok along with the remaining tamari and the beef strips. Cook for a further 1 min.
Spoon the stir-fry into bowls. Tear up the coriander leaves roughly and sprinkle on top. Serve straight away.