• Gather all the ingredients. If you prefer to make the sauce from scratch, see my Hayashi Rice recipe.
  

To Prepare the Ingredients

• Cut 1½ onions into thin wedges.
  
• Trim the stem ends from 4 cremini mushrooms and cut the caps into thin slices.
  
• Cut 10 oz thinly sliced beef (chuck or ribeye) into 2-inch (5 cm) sections.
  

To Cook

• Heat a medium pot over medium heat. When it's hot, add 1 Tbsp unsalted butter. When it's melted, add the onion and sprinkle with Diamond Crystal kosher salt. Sauté for 5 minutes until slightly wilted.
  
• Then, add the beef to the pot. Season with Diamond Crystal kosher salt and freshly ground black pepper.
  
• Stir and cook for 2–3 minutes until the meat is no longer pink.
  
• Add the mushrooms and stir to combine. Then, add 1¾ cups water.
  
• Increase the heat to high, cover the pot, and bring it to a boil. Once boiling, lower the heat to maintain a simmer and cook for 20 minutes. Occasionally skim off the scum and fat floating on the stock's surface to keep the liquid clear and clean.
  

Add the Roux

• Turn off the heat. Put 1–2 cubes from ½ box Hayashi Rice Sauce (Roux) in a ladleful of cooking liquid. Slowly stir it with a spoon or chopsticks until dissolved, then release it to the pot and stir to incorporate. Repeat with the rest of the roux, 1–2 cubes at a time. 
  
• Turn on the heat to medium and cook for another 5 minutes, stirring occasionally. Divide 4 servings cooked Japanese short-grain rice on one side of shallow serving bowls (I used this Tuxedo Cat Salad Plate from JOC Goods) . Ladle the hayashi rice sauce on the other side. Enjoy!
  

To Store

• Keep the leftovers in an airtight container in the refrigerator for 3 days or freezer for up to a month.