Wash the potatoes under cold running water.
Peel them with a vegetable peeler and remove the eyes.
Cut each potato into four equal pieces.
Put the water and potatoes in a large pot, cover, and bring to a boil over medium heat, leaving the lid slightly ajar to prevent the pot from boiling over.
Reduce the heat to maintain a gentle boil and cook until done.
Mince one onion finely.
Preheat a large skillet on medium heat, then add one tablespoon of oil.
Sauté the onions until translucent and tender; make sure there is no moisture left in the onions so that the croquettes don‘t get soggy.
Add the ground beef to the pan and break it up with a wooden spoon.
Season the mixture with salt and pepper.
Stir to combine and cook until the meat is no longer pink.
Turn off the heat and remove the pan from the stove to let it cool slightly.
Once in a while, check if the potatoes are done cooking. Insert a skewer into a bigger piece of potato; if it pierces the potato easily, it‘s done.
Remove the potatoes from the heat and drain off the water completely.
Move the pot back to the stove. Shake the pot over low heat and let the remaining moisture completely evaporate (without burning the potatoes).
Transfer the potatoes to a large bowl and mash them with the butter and some more salt and pepper while they are still hot.
Gently squeeze the liquid from the meat mixture by collecting it to one side of the pan.
Add the drained meat mixture to the bowl with the mashed potatoes and mix to combine.
While the mixture is still warm (but not hot), form it into oval-shaped patties roughly three inches in length, taking care not to create air pockets in the patties.
Place the patties on a tray or plate, cover, and let them rest in the fridge for 15–30 minutes.
Prepare one tray each for the flour and the panko.
Crack three large eggs into a small bowl and beat with a fork.
Dredge each patty in flour and shake off the excess.
Then, dip each patty in the egg and coat with the panko, pressing the breadcrumbs to make sure they adhere.
Once you finish breading the patties, add roughly four cups of oil to a pot. Make sure there‘s at least two inches in the pot so the croquettes are completely submerged.
Bring the oil to 170–180ºC over medium heat. Use a cooking thermometer to check the oil temperature (so there‘s no guessing!)
Deep-fry 2–3 croquettes at a time until they are golden brown, about 2–3 minutes.
Transfer the croquettes to a wire rack or a plate lined with paper towels to drain the excess oil.
Serve the Korokke with tonkatsu sauce.
You can 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.
