• Gather all the ingredients.
  

To Make the Tartar Sauce

• Place 1 large egg (50 g each w/o shell) in a saucepan and cover with water by one inch. Bring the water to a boil on medium heat. Once the water boils, lower heat to maintain a gentle boil (but don’t let the egg jump around) for the next 12 minutes.
  
• Remove the egg from the boiling water and submerge it in iced water to stop the cooking. When the egg is cool, peel the shell and set the egg aside.
  
• Mince ¼ onion as finely as possible: Lay the onion flat side down on the cutting board. With the tip of a very sharp knife pointing toward the root end, make ⅛-inch vertical slices to within ½ inch of the root end. Next, rotate the onion 90 degrees and cut 2–3 horizontal slices toward the root end, again keeping the root intact.
  
• Finally, cut ⅛-inch slices perpendicular to the previous slices you made. To finely mince, run your knife through the cut onions in a rocking motion. Put the finely minced onion in cold water for 5–10 minutes to remove the sharp, pungent, and astringent taste. 
  
• Cut off the ends of ½ Japanese or Persian cucumber and slice in half lengthwise. Using a spoon, remove the seeds to help get rid of excess moisture from the cucumber.
  
• Cut the cucumber halves into thin strips lengthwise.
  
• Then, finely mince the strips into ⅛-inch pieces. Transfer the minced cucumber to a medium bowl. Here, I used 3 heaping tablespoons of finely chopped cucumber for two servings. I reserved the leftovers as a topping for a cold udon noodle dish.
  
• Drain the onion in a fine-mesh colander, squeezing the excess moisture from the onion pieces. Next, add them to the bowl with the cucumber. You should have roughly the same amount of cucumber and onion.
  
• Mince the boiled egg, especially the egg white, and then transfer it to the bowl.
  
• Add 3 Tbsp Japanese Kewpie mayonnaise, 1 tsp rice vinegar (unseasoned), ⅛ tsp Diamond Crystal kosher salt, and ⅛ tsp freshly ground black pepper, and mix them all together. Cover the bowl with plastic and refrigerate until serving.
  

To Make the Nanban Sauce

• In a medium saucepan, combine 2 Tbsp soy sauce, 2 Tbsp rice vinegar (unseasoned), 1 Tbsp mirin, and 2 Tbsp sugar. Bring the mixture to a gentle simmer on medium heat. Once the sugar is completely dissolved, turn off the heat and set aside.
  

To Make the Fried Chicken

• Butterfly 1 boneless, skinless chicken breast by splitting it horizontally with a knife, stopping before you cut all the way through, opening it like a book. When you open the breast, it has two matching sides, resembling a butterfly.
  
• Cut the butterflied chicken breast in half down the middle to create cutlets. Pound both sides of the meat with the back of the knife or a meat tenderizer/mallet.
  
• Season both sides of the cutlets with ⅛ tsp Diamond Crystal kosher salt and ⅛ tsp freshly ground black pepper and coat them with 1 Tbsp all-purpose flour (plain flour), dusting off any excess. Then, set aside. If the chicken cutlets are still cold from the refrigerator, leave them on the counter for 10 minutes or so while the oil is heating up.
  
• Add 2 cups neutral oil to a medium pot (I used this carbon steel pot) and bring it to 340ºF (170ºC) over medium heat. If you don’t have an instant-read thermometer, stick a chopstick in the oil; if tiny bubbles start to appear around the tip of the chopstick. the oil is ready.
  
• While heating the oil, crack 1 large egg (50 g each w/o shell) in a shallow tray or container and beat it until thoroughly blended. When the oil reaches 340ºF (170ºC), dip one floured cutlet into the egg and coat well.
  
• First deep-frying: Gently put one piece of chicken into the oil and cook for 4 minutes, flipping the chicken once at the halfway mark. Don‘t touch or flip the chicken for the first minute until the egg coating is solidified. Why do I deep-fry only one piece at a time? I do this so the oil temperature doesn’t drop drastically. Keep watching the oil temperature and make sure it doesn’t go over 340ºF (170ºC) or else the chicken will get dark too quickly while the inside will be undercooked. Scoop and discard any fried crumbs in the oil with a fine-mesh strainer so the oil doesn’t get darker and dirtier with burnt crumbs. After 4 minutes, transfer the chicken to a wire rack or paper towel and let it sit to remove the excess oil while you fry the next cutlet. The hot oil on the exterior will slowly continue to cook the meat as it rests. Continue to coat the remaining cutlet(s) in egg and deep-fry.
  
• Second deep-frying: Raise the oil temperature to 350ºF (180ºC) and carefully place the first piece of fried chicken in the oil again. Be careful of bigger oil splatters than during the first round of frying. This happens when the moisture below the crust of the fried chicken vaporizes and hits the hot oil, causing it to splatter. Fry the chicken again for 30 seconds on each side.
  
• Remove the chicken and place on the wire rack or paper towel and let it sit to remove excess oil. If you have the instant-read thermometer, check to make sure the internal temperature of the chicken is at least 165ºF (74ºC). Repeat the second frying with the remaining cutlet(s).
  
• Reserve 2 tablespoons of the Nanban Sauce in a little bowl or cup for serving. Transfer the rest of the sauce into a small shallow and rimmed tray.
  
• While the chicken cutlets are hot, coat them with the Nanban Sauce.
  

To Serve

• Carefully slice each fried cutlet into pieces ¾ inch (2 cm) wide and transfer to individual plates. Here, I serve the chicken with a simple, leafy salad with sesame dressing in a small cup on the side and tomato wedges (optional). Place a generous amount of the Tartar Sauce on top of the fried chicken. Drizzle extra Nanban Sauce on top of the chicken and enjoy.
  

To Store

• You can store the leftover chicken in an airtight container in the refrigerator for three days or in the freezer for up to a month. However, I do not recommend keeping the tartar sauce in the freezer. Please make a new batch of the Tartar Sauce and Nanban Sauce when you serve any leftover fried chicken. To reheat, put the defrosted or frozen fried chicken on a baking sheet lined with aluminum foil or parchment paper. Bake at 350ºF (180ºC) for 15–20 minutes if defrosted, or 30–40 minutes if frozen. Ensure that the chicken is thoroughly warmed through before serving.