Dice the red onion small and sauté in a little butter - deglaze with a dash of fish sauce and pour the broth on top - heat on medium heat - season with the soy sauce until it is salty enough for you.
Leave on the stove at the lowest setting. Cook the rice noodles and rinse in cold water. Meanwhile, remove the mushrooms from the stem - cut the lime in half and the green spring onions into strips
Briefly sear the steak on all sides and then cut it into thin slices of the same size (the meat cooks in the broth - but I just like it crispy on the outside). To serve: first the noodles, then the broth, then gradually add the fresh ingredients - Pour the lime juice over it before eating.