• Please note that the hamburger patties require 30 minutes of chilling time before cooking. For the steamed rice, 1½ cups (2 rice cooker cups, 300 g) of uncooked Japanese short-grain rice yield roughly 4 servings (4⅓ cups, 660 g) of cooked rice. See how to cook it with a rice cooker, pot over the stove, Instant Pot, or donabe.

  Now, gather all the ingredients.
  

To Sauté the Onion

• Mince 1 onion finely. I use the mijingiri cutting technique.
  
• Heat 1 Tbsp extra virgin olive oil in a large pan on medium-high heat and sauté the onion until translucent.
  
• Turn off the heat. Divide the sautéed onion to use for the gravy and the hamburger patties.
  

To Make the Gravy

• Cut 6 button mushrooms into thin slices. Heat a small saucepan on medium heat. Then, add ½ Tbsp extra virgin olive oil and the mushrooms and sauté until soft. Add half of the sautéed onion and stir together.
  
• Add 1½ cups beef stock/broth, 1 tsp Worcestershire sauce, and 1 tsp soy sauce.
  
• Skim off the scum and foam from the surface with a fine-mesh skimmer. Then, season with Diamond Crystal kosher salt to taste and ⅛ tsp freshly ground black pepper.
  
• Make a slurry with 1 Tbsp potato starch or cornstarch and 1 Tbsp water in a small bowl and whisk well to combine. Pour it into the gravy and thoroughly mix together. Simmer until the gravy thickens, then turn off the heat and set aside.
  

To Make the Patties

• Add ½ lb ground beef, ¼ lb ground pork, and the other half of the sautéed onion to a large bowl. Add ⅓ cup panko (Japanese breadcrumbs), 1 large egg (50 g each w/o shell), 2 Tbsp milk, 1 tsp Worcestershire sauce, ½ tsp nutmeg, 1 tsp Diamond Crystal kosher salt, and ⅛ tsp freshly ground black pepper.
  
• Knead together well using your hand until the mixture becomes sticky and pale. Then, divide into 4 portions for large patties or 6 portions for medium patties.
  
• Toss a portion from one hand to the other about 5 times; this releases the air inside and prevents cracking during cooking. Then, make an oval-shaped patty with a slightly rounded top. Place on a plate or tray. Repeat with the other portions. Cover with plastic wrap and rest them in the refrigerator for at least 30 minutes so that the meat melds and the fat solidifies.
  

To Cook the Hamburger Steaks

• Heat a large pan over medium heat, then add ½ Tbsp extra virgin olive oil. Place the patties gently in the pan. (Fry in batches if you‘re doubling the recipe.) Indent the center of each patty with 2 fingers because the meat will rise with the heat. Fry them for about 5 minutes or until browned on the bottom. Flip over, then cover with a lid and fry until thoroughly cooked inside, about 5 minutes (depending on the thickness of the patties). At this time, reheat the gravy and make the fried eggs with 4 large eggs (50 g each w/o shell) and ½ Tbsp extra virgin olive oil.
  

To Serve

• Divide 4 servings cooked Japanese short-grain rice on individual plates. Transfer the patties to the plates, pour the gravy on top, and top each with a fried egg. Chop 1 sprig parsley and sprinkle on top for garnish. Serve immediately.
  

To Store

• You can keep the leftovers in an airtight container and store in the refrigerator for 3 days or in the freezer for a month.