What happens next?

How to make gyudon
Slice the beef and vegetables.
Use a sharp knife to shave the beef into very thin slices. Cut the onion and shiitake mushrooms into moderately thin slices, as well.
To save yourself time and effort, ask the butcher to slice the beef very thinly before packaging it. If the butcher does not slice the beef, freeze it for 1 hour before slicing it yourself.