• Gather all the ingredients.
  

To Make the Tartar Sauce

• Place 1 large egg (50 g w/o shell) in a saucepan. Cover with water by one inch. Turn on the heat to medium. Bring the water to a boil. Lower the heat to maintain a gentle simmer for the next 12 minutes (don’t let the egg bounce around).
  
• Remove from the heat. Submerge the egg in iced water to stop the cooking. When cool enough to handle, peel and set aside.
  
• Mince ¼ onion as finely as possible. Place it cut side down. Make ⅛-inch (3 mm) vertical slices to within ½ inch of the root end. Cut 2–3 horizontal slices toward the root.
  
• Slice crosswise ⅛ inch (3 mm) for an even mince. Mince finer by running your knife through them in a rocking motion. Submerge the onion in cold water for 5–10 minutes to remove the sharpness.
  
• Cut off the ends from ½ Japanese or Persian cucumber. Slice in half lengthwise. Remove the seeds with a spoon.

  Nami's Tip: Taking out the seeds helps get rid of excess moisture.
  
• Cut the cucumber halves lengthwise into thin strips.
  
• Finely mince them crosswise into ⅛-inch (3 mm) pieces. Place in a medium bowl. I used 3 heaping tablespoons of finely chopped cucumber for two servings.

  Nami's Tip: Reserve the leftover cucumber to use in a salad or as topping for cold udon noodles.
  
• Drain the onion in a fine-mesh colander. Squeeze out the excess moisture. Add them to the bowl with the cucumber.

  Nami's Tip: Use roughly the same amount of cucumber and onion.
  
• Mince the boiled egg, especially the egg white. Add to the cucumber and onion.
  
• Add 3 Tbsp Japanese Kewpie mayonnaise, 1 tsp rice vinegar (unseasoned), ⅛ tsp Diamond Crystal kosher salt, and ⅛ tsp freshly ground black pepper. Mix well. Cover the bowl and refrigerate until serving.
  

To Make the Nanban Sauce

• Combine 2 Tbsp soy sauce, 2 Tbsp rice vinegar (unseasoned), 1 Tbsp mirin, and 2 Tbsp sugar in a medium saucepan. Place over medium heat. Bring the mixture to a gentle simmer. Mix well until the sugar is dissolved. Turn off the heat. Set aside.
  

To Prepare the Cutlets

• Butterfly (split) 1 boneless, skinless chicken breasthorizontally with a knife, stopping before you cut all the way through. Open it like a book. It will have two matching sides like butterfly wings.
  
• Cut in half down the middle to get two cutlets. Pound both sides with the back of the knife or a meat tenderizer/mallet.
  
• Season both sides with ⅛ tsp Diamond Crystal kosher salt and ⅛ tsp freshly ground black pepper. Coat with 1 Tbsp all-purpose flour (plain flour). Dust off any excess. Set aside.

  Nami's Tip: If the cutlets are still cold from the refrigerator, leave them on the counter for 10 minutes or so while the oil is heating up in the next step.
  

To Fry the Chicken

• Add 2 cups neutral oil to a medium pot (I used a carbon steel pot). Turn on the heat to medium. Heat the oil to 340ºF (170ºC).

  Nami's Tip: I use an instant-read thermometer for accuracy. If you don't have one, stick a chopstick in the oil; when tiny bubbles appear around the tip, the oil is ready.
  
• Meanwhile, crack 1 large egg (50 g w/o shell) in a shallow tray. Beat until thoroughly blended. When the oil reaches 340ºF (170ºC), dip one floured cutlet into the egg and coat well.

  Nami's Tip: We deep-fry only one piece at a time. Why? So the oil temperature doesn’t drop drastically. Likewise, don't let the temperature go higher than 340ºF (170ºC). Otherwise, the chicken will darken too quickly while the inside stays undercooked.
  
• FIRST FRY: Gently lower one piece of chicken into the hot oil. Cook for 4 minutes total, flipping it over at the halfway mark. Don‘t touch the chicken for the first minute to allow the egg coating to solidify. Scoop and discard any fried crumbs in the oil with a fine-mesh strainer.

  After 4 minutes, transfer the chicken to a wire rack or paper towel to drain excess oil. The residual heat will continue to cook the meat as it rests. Meanwhile, fry the next cutlet and transfer it to a wire rack.

  Nami's Tip: Cleaning the oil keeps it from getting darker and dirtier with burnt crumbs.
  
• SECOND FRY: Raise the oil temperature to 350ºF (180ºC). Carefully place the first piece of fried chicken in the oil again. Fry for 30 seconds on each side.

  Nami's Tip: Be careful of bigger oil splatters than during the first fry. This happens when the moisture underneath the crust vaporizes and hits the hot oil.
  
• Remove the chicken from the oil. Place on the wire rack or paper towel to drain excess oil. If you have an instant-read thermometer, check that the chicken's internal temperature is at least 165ºF (74ºC). Repeat frying the next cutlet.
  

To Coat with Nanban Sauce

• Reserve 2 tablespoons of the nanban sauce in a small bowl for serving. Add the rest of the sauce to a tray.
  
• While the chicken cutlets are hot, add them to the tray. Coat them with the sauce using a spoon.
  

To Serve

• Slice each fried cutlet crosswise into ¾-inch (2-cm) pieces. Transfer to individual plates. Dollop a generous amount of tartar sauce on top. Drizzle with extra nanban sauce. Enjoy!
  

To Store

• Store the leftover chicken in an airtight container. Keep in the refrigerator for 3 days or in the freezer for up to a month. I do not recommend freezing the tartar sauce. Reheat defrosted or frozen fried chicken on a baking sheet lined with aluminum foil or parchment paper. Bake until warmed through at 350ºF (180ºC) for 15–20 minutes if defrosted, or 30–40 minutes if frozen. Make a new batch of tartar sauce and nanban sauce when you serve leftover fried chicken.