• Gather all the ingredients.
  

To Make Patties

• Peel and cut potatoes into thirds or half and place in a large pot. Add water until it covers the potatoes by 1 inch (2.5 cm). Bring it to a boil and cook the potatoes until a skewer goes through easily, this will take about 20 minutes depending on the amount of water.
  
• While cooking the potatoes, you can finely dice the onion.
  
  
• In a large fry-pan, heat 1 Tbsp oil on medium-high heat and sauté the onion until soft. Then add the meat and cook until no longer pink.
  
• Season with freshly ground black pepper and ¼ tsp salt. Remove from the heat and set aside.
  
• Combine the panko and 2 Tbsp oil in a separate frying pan and toast over medium heat until golden brown. Transfer the panko into a bowl or shallow dish and allow to cool.
  
• When a skewer goes through the potatoes easily, transfer the potatoes to a large bowl and drain.
  
  
• Mash the potatoes and season with freshly ground black pepper, ½ tsp salt, and 1 Tbsp butter.
  
• Add the meat mixture into the mashed potatoes and combine well. Some onions and meat yield more juice after cooking. In that case, avoid adding any juice into the potatoes. Wet potatoes will make it harder to shape the croquettes.
  
  
• While the mixture is still warm, but not hot, make thick oval patties, about ½ inch thick. Let the patties rest in the refrigerator for 15-30 minutes.
  

To Bread the Croquettes

• Preheat the oven to 400ºF (200ºC). For a convection oven, reduce cooking temperature by 25ºF (15ºC). Coat the patties with flour and pat off any excess flour.
  
• Then coat the patties with eggs and finally panko.
  

To Bake

• Place the croquettes on a baking sheet lined with a wire rack (so the bottom of croquettes will be crunchy) or parchment paper. Bake at 400ºF (200ºC) for 10 minutes. Serve immediately with Tonkatsu sauce.
  

To Store

• In order to store croquettes, please bake all the croquettes first since you use raw eggs. Let cool completely, put them in an airtight container, and store in the freezer for up to a month. To reheat, thaw them in the refrigerator overnight, put half defrosted (or frozen if you didn't thaw in advance) croquettes on a baking sheet lined with parchment paper and bake at 350ºF (180ºC) for 15-45 minutes (based on how defrosted they are) or until the inside is warm. I don't recommend microwaving because croquettes will explode. You can only microwave for a very short time, but finish in the oven for a crispy shell and hot on the inside.=