Cut the beef, onion, and mushrooms into bite-sized pieces.
Heat cooking oil in a pan and stir-fry the beef.
When the beef is cooked, add the onions and mushrooms and stir-fry together.
Add flour and mix well with the ingredients.
Pour in water and Hayashi rice sauce, and bring to a boil.
Turn off the heat when all ingredients are cooked and the sauce thickens.
Serve the finished Hayashi rice over steamed rice.