• Gather all the ingredients.
  

To Boil the Potatoes

• Wash 2 lb russet potatoes under cold running water. Peel them and remove the eyes. Tip: I use russets for their starchy, fluffy, and light texture. They are the best variety for soaking up the juices and flavor from the meat and onion.
  
• Cut each potato into 4 equal pieces. Tip: Keep the pieces a similar size so they cook evenly.
  
• Put the water and potatoes in a large pot and cover with the lid slightly ajar to prevent boiling over. Turn on the heat to medium and bring it to a boil; this will take about 15 minutes. Reduce the heat to maintain a gentle boil and cook until done, about 15–20 minutes.
  

To Cook the Onions and Meat

• Meanwhile, prepare the onion and ground beef. Mince 1 onion finely (called mijingiri in Japanese). Lay an onion half on the cutting board, flat side down. Make ¼-inch vertical slices to within ½ inch of the root end. Then, make ¼-inch horizontal slices, again keeping the root intact.
  
• Make perpendicular cuts down through the vertical slices. If you need to chop the onions finer, run your knife through them using a rocking motion while holding down the tip of the knife.
  
• Preheat a large skillet on medium heat, then add 1 Tbsp neutral oil. Then, add the minced onions.
  
• Sauté until translucent and tender, about 12–15 minutes. Leave the onions for a long stretch between stirring to give them a nice char and take care not to burn them. Tip: Make sure to evaporate the moisture in the onions; any extra liquid will make the croquettes soggy.
  
• Add 1 lb ground beef to the pan and break it up with a wooden spoon.
  
• Season with ½ tsp Diamond Crystal kosher salt and ¼ tsp freshly ground black pepper.
  
• Stir and cook until the meat is no longer pink. Turn off the heat and let it cool slightly.
  

To Combine the Potatoes and Meat

• To check if the potatoes are cooked, insert a skewer into a bigger piece; it's done if it pierces easily. Remove from the heat and pour off all the water. I use a lid to hold back the potatoes so they don't fall out.
  
• Move the pot back to the stove over low heat. Continuously shake the pot to evaporate the remaining moisture from the potatoes, about 2–3 minutes. The potatoes should look powdery and dry. Transfer them to a large bowl.
  
• Mash the potatoes with a potato masher while they are still hot, allowing the steam to escape. Keep some potato chunks for texture. Then, add 1 Tbsp unsalted butter.
  
• Add ½ tsp Diamond Crystal kosher salt and ¼ tsp freshly ground black pepper and combine gently.
  
• Gently squeeze the liquid from the meat mixture by collecting it to one side of the pan. We do this to avoid adding moisture to the potatoes.
  
• Add the drained meat to the bowl with the potatoes and mix gently to combine.
  

To Make the Patties

• While the mixture is still warm (but not hot), form oval-shaped patties 3 inches (7.5 cm) in length. Try not to create air pockets that can make the patties explode while deep-frying.
  
• Placed the patties on a tray or plate and cover. Rest them in the refrigerator for 15–30 minutes. Do not skip this step! Tip: Cooling the patties melds the ingredients and prevents the croquettes from exploding while cooking. Warm patties will get hot inside and create steam that will puncture a hole in the panko coating and explode. Cold patties will not create and release steam.
  

To Bread the Croquettes

• Prepare separate trays for ½ cup all-purpose flour (plain flour) and 2 cups panko (Japanese breadcrumbs). Crack 3 large eggs (50 g each w/o shell) into a small bowl and beat with a fork.
  
• Dredge each patty in flour and shake off the excess.
  
• Dip in the beaten egg, then coat with the panko, pressing to adhere the breadcrumbs.
  

To Deep-Fry

• Next, add 4 cups neutral oil to a medium pot. Make sure there‘s at least 2 inches (5 cm) of oil in the pot so the croquettes are completely submerged while cooking.
  
• Bring the oil to 340–350ºF (170–180ºC) over medium heat. Use a cooking thermometer to check the oil temperature so there‘s no guessing. Tip: Once you get used to deep-frying, you can use the chopstick or panko tricks to determine if the oil is ready.
  
• Deep-fry 2–3 croquettes at a time until golden brown, about 2–3 minutes. Do not touch or flip the croquettes until one side is golden brown. The inside is already cooked, so you just need to fry the breading. Tip: Do not crowd the pot as the oil temperature will drop quickly and cause the food to absorb more oil.
  
• Transfer to a wire rack or a plate lined with paper towels to drain the excess oil. Continue deep-frying the rest of the patties.
  

To Serve

• Serve with tonkatsu sauce (bottled or Homemade Tonkatsu Sauce). In Japan, we typically serve deep-fried foods with shredded green cabbage. I shred it with a julienne slicer or stainless steel cabbage slicer and serve it with Japanese Sesame Dressing. I also add a few wedges of tomato for color.
  

To Store

• Store the leftovers in an airtight container and freeze up to a month. To reheat, put the defrosted or frozen croquettes on a baking sheet lined with aluminum foil or parchment paper. Bake at 350ºF (180ºC) for 15–20 minutes if defrosted or 45 minutes if frozen. Check if the inside is warm before serving.