1. Wash 2 lb russet potatoes under cold running water. Peel them with a vegetable peeler and remove the eyes. Tip: Use russet potatoes for their starchy, fluffy, and light texture. They are the best variety at soaking up the juice and flavor from the meat and onion.
2. Cut each potato into equal 4 pieces. Tip: To cook evenly, it‘s important that they are of similar size.
3. 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. It will take about 15 minutes to come to a boil. Reduce the heat to maintain a gentle boil and cook until done, about 15–20 minutes.
4. Prepare the rest of the ingredients while you wait for the potatoes to cook. First, mince 1 onion finely (we call this cutting technique mijingiri in Japanese). Cut the onion in half. Lay one half flat side down on the cutting board. With the knife tip pointing toward the root end, make 1/4-inch vertical slices to within 1/2 inch of the root end. Then, with the knife edge toward the root end, make 1/4-inch horizontal slices, again keeping the root intact.
5. Finally, make perpendicular cuts down through the vertical slices you made. Repeat with the other onion half. If you need to chop the onions finer, run your knife through them using a rocking motion. Hold down the tip of the knife; otherwise, the onions will go flying around the room.
6. Preheat a large skillet on medium heat, then add 1 Tbsp neutral oil. Then, add the minced onions.
7. Sauté the onions until translucent and tender, about 12–15 minutes. Take care not to burn the onions. Tip: Make sure there is no moisture left in the onions so that the croquette don‘t get soggy. Leave the onions for a long stretch between stirring to give them a nice char.
8. Add 1 lb ground beef to the pan and break it up with a wooden spoon.
9. Season the mixture with 1/2 tsp Diamond Crystal kosher salt and 1/4 tsp freshly ground black pepper.
10. 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.
11. 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. When you do so, use a lid to keep the potatoes from falling out of the pot.
12. Move the pot back to the stove. Shake the pot over low heat and let the remaining moisture completely evaporate (without burning the potatoes), about 2–3 minutes. Then, transfer them to a large bowl.
13. With a potato masher, mash the potatoes while they are still hot, allowing the steam to escape in the process. Add 1 Tbsp unsalted butter. Tip: Keep some potato chunks for texture.
14. Add 1/2 tsp Diamond Crystal kosher salt and 1/4 tsp freshly ground black pepper and combine well.
15. Gently squeeze the liquid from the meat mixture by collecting it to one side of the pan. We do this to avoid adding excess moisture to the potatoes.
16. Add the drained meat mixture to the bowl with the mashed potatoes and mix to combine.