• Gather all the ingredients. If doubling the recipe, please cook one batch at a time in the pan/pot.
  

To Prepare the Ingredients

• Finely mince ½ onion for the hambagu patties. Lay the onion half flat side down on the cutting board. With the knife tip pointing toward the root end, make ⅛-inch vertical slices to within ½ inch of the root end. Then, with the knife edge toward the root end, make ⅛-inch horizontal slices, again keeping the root intact.
  
• Finally, make perpendicular cuts down through the vertical slices you made. If you need to chop the onions finer, run your knife through them using a rocking motion. Be sure to hold down the tip of the knife; otherwise, the onions will go flying around the room. Set aside.
  
• Thinly slice ½ onion for the hambagu sauce (keep divided if you're doubling the recipe). Set aside.
  
• Cut off and discard the bottom of 1 package shimeji mushrooms and separate them into small clusters (keep divided if you‘re doubling the recipe). Set aside for the hambagu sauce.
  

To Sauté the Minced Onion for the Patties

• Heat a large frying pan over medium heat. Add 1 Tbsp unsalted butter while the pan is heating up and distribute it evenly. Don‘t burn the butter. Add the minced onion to the pan.
  
• Sauté the minced onion until golden brown and translucent.
  
• When the onions are done cooking, transfer to a large bowl and let cool.
  

To Shape the Patties

• To the sautéed minced onions, add ¾ lb ground beef and pork combination, ½ tsp Diamond Crystal kosher salt, ⅛ tsp freshly ground black pepper, ½ tsp nutmeg, ⅓ cup panko (Japanese breadcrumbs), 1 large egg (50 g each w/o shell), and 2 Tbsp milk.
  
• Using a silicone spatula or your hand, mix it all together until well combined. Then, knead the mixture, in a circular motion, until it gets sticky and becomes pale in color. Your arm may get tired, but it‘s a very important step to meld the ground meats together.
  
• Divide the mixture into quarters and scoop one portion into your hand.
  
• Toss it from one hand to the other repeatedly about five times in order to release any air inside the mixture (see the video link above). Tip: Without releasing the air inside, the hamburger steaks will likely crack while cooking.
  
• Make an oval-shaped patty and place it on a tray or plate. Repeat with the remaining portions. Cover the patties with plastic wrap and refrigerate them for at least 20–30 minutes before cooking so that the flavors of the meat further meld together and the fat solidifies.
  

To Make the Sauce

• While the hambagu patties are resting, make the sauce. (Cook one batch at a time if you‘re doubling the recipe.) On the stove over medium heat, set a large pot (I used this 5.5-QT Staub cast iron pot) that fits 4 patties. When it‘s hot, add 1 Tbsp unsalted butter and 1 Tbsp neutral oil. Distribute them evenly and add the onion slices.
  
• Using the blunt-end wooden spatula, sauté the onion slices until tender and translucent.
  
• Add the shimeji mushrooms and toss to coat with the oil and butter. Cook until they become slightly tender.
  
• Next, start making the roux: Turn the heat to low and sprinkle in 1 Tbsp all-purpose flour (plain flour). Cook the flour with the onions and mushrooms for 2–3 minutes, scraping the bottom of the pot constantly. It‘s important that you cook the flour gently during this step to get rid of its raw taste, but don‘t let it toast or turn dark. You want the pale color of a white roux. If it starts to brown, turn off the heat temporarily while you finish cooking it. Once it‘s done cooking, turn the heat to medium low.
  
• Next, add about 1–2 Tbsp of chicken stock/broth to the pot. Use your spatula to quickly and vigorously blend the stock well with the flour mixture. You will notice the flour absorb all the moisture and clump up.
  
• Continue to add 1–2 Tbsp of stock at a time, stirring quickly after each addition to combine it well with the flour and smooth out any clumps. Make sure there are no lumps of flour left when you‘re done adding all the stock. Your sauce base is now done.
  
• Next, add ¼ cup red wine, ¼ cup ketchup, and ¼ cup Worcestershire sauce to the pot.
  
• Add 1 bay leaf and 2 tsp sugar and mix well together. Increase the heat to medium and bring it to a gentle simmer. Once simmering, turn off the heat.
  

To Sear the Patties

• Take out the hambagu patties from the refrigerator. (Cook one batch at a time if you‘re doubling the recipe.) Heat a large frying pan over medium heat. When it‘s hot, add 1 Tbsp neutral oil and distribute it evenly. Place the patties gently into the pan, spacing them 1 inch (2.5 cm) apart so you can easily insert the spatula to flip the patties later on.
  
• Indent the center of each patty with 2–3 fingertips because the centers will rise with the heat. Cook the patties until browned on the bottom, about 3 minutes.
  
• Then, carefully flip and cook the other side for another 3 minutes or until browned. Turn off the heat. Tip: The inside of the patties won‘t be cooked through yet. You‘ll finish cooking the hambagu in the sauce next.
  

To Cook the Nikomi Hambagu

• Gently transfer the 4 patties and their pan juices into the pot with the hambagu sauce.
  
• Close the lid and bring it to a gentle simmer on medium heat. Once simmering, reduce the heat to medium low. Open the lid and spoon the sauce over the patties.
  
• Close the lid again and simmer gently for 15 minutes. Occasionally uncover the pot to coat the patties with the sauce. You do not need to flip them. After 15 minutes of cooking, the Nikomi Hambagu is ready to serve.
  

To Serve

• Serve the hambagu hot on individual plates along with the hambagu sauce. I typically serve the dish with oven-baked potato wedges, blanched broccoli, butter-sautéed carrots, or/and corn kernels. You can also bring the pot to the table to keep it hot. Here, I sprinkled them with dried parsley for color; you can use fresh chopped parsley as well.
  

To Serve the Nikomi Hambagu in Individual Mini Cast Iron Pans

• Nikomi hambagu is often served hot in an individual cast iron pan (I used this 6½-inch cast iron skillet). Place the Nikomi Hambagu, sauce, blanched broccoli, and oven-baked potato wedges in the skillet and heat it until the sauce is hot and bubbly.
  
• You can serve the Nikomi Hambagu with 4 slices provolone cheese. I placed one slice of provolone cheese on top of each hot hambagu and let it melt. Serve immediately.
  

To Store

• You can keep the leftovers in an airtight container and keep them for 3–4 days in the refrigerator or for up to a month in the freezer.