• Gather all the ingredients.

  Make the tartar sauce first so it has time to chill in the refrigerator—it tastes best served cold alongside the hot fried chicken.
  

To Make the Tartar Sauce

• Place 1 large egg (50 g w/o shell) in a saucepan and cover with water by 1 inch (2.5 cm). Bring to boil over medium heat, then reduce to a gentle simmer for 12 minutes until fully hard-boiled.
  
• Transfer the egg to an iced bath to stop the cooking. Once 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.

  Nami's Tip: Soaking the onion removes 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: Scooping out the seeds keeps excess moisture out of the tartar sauce.
  
• Cut the cucumber halves lengthwise into thin strips.
  
• Finely mince them crosswise into ⅛-inch (3 mm) pieces. Place in a medium bowl.

  Nami's Tip: Add as much or as little as you like—more for a chunkier, fresher sauce, less for a creamier one. I use about 4 tablespoons. Reserve any leftover cucumber for salads or cold udon noodles.
  
• Drain the onion in a fine-mesh colander. Squeeze out the excess moisture and add to the bowl with the cucumber.
  
• 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 until smooth and well combined. 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. Bring to a gentle simmer over medium heat. Stir until the sugar dissolves completely, then remove from the heat.
  

To Prepare the Cutlets

• Butterfly (split) 1 boneless, skinless chicken breast: Cut horizontally with a knife, stopping ½ inch before cutting all the way through, then open flat like a book.

  Nami's Tip: If the chicken is still cold from the refrigerator, leave it on the counter for 10–15 minutes. Cold chicken lowers the oil temperature too quickly, leading to uneven cooking.
  
• Cut in half down the middle to get two cutlets. Pound both sides with the back of a knife or a meat mallet until the cutlets are an even thickness throughout.
  
• 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.
  

To Fry the Chicken

• Add 2 cups neutral oil to a medium pot. Bring it to 340ºF (170ºC) over medium heat.

  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) into a shallow tray and beat until thoroughly blended. Dip one floured cutlet into the egg, turning to coat both sides completely.
  
• FIRST FRY: Gently lower one chicken cutlet into the hot oil. Leave it undisturbed for the first minute so the egg coating can set, then cook for 4 minutes total, flipping halfway. Transfer to a wire rack to drain.

  Between batches, scoop out any fried crumbs with a fine-mesh strainer to keep the oil from getting dark and bitter.

  Fry the second cutlet and transfer it to the rack.

  Nami's Tip: Fry one cutlet at a time to keep oil temperature stable—and don't climb above 340ºF (170ºC), or the outside will darken before the inside cooks through.
  
• SECOND FRY: Raise the oil temperature to 350ºF (180ºC). Carefully place the first chicken cutlet in the oil and fry again for 30 seconds on each side, until deep golden and crisp.

  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 and transfer to a wire rack or paper towel to drain. Repeat with the second cutlet.

  Nami's Tip: If you have an instant-read thermometer, check that the chicken's internal temperature is at least 165ºF (74ºC). The chicken will continue cooking from residual heat as it rests.
  

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.
  
• Add the hot chicken cutlets to the tray immediately and spoon the nanban sauce over both sides until all surfaces are evenly coated.
  

To Serve

• Slice each cutlet crosswise into ¾-inch (2-cm) pieces. Transfer to individual plates and top with a generous spoonful of tartar sauce. Drizzle with the reserved nanban sauce.
  

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. Do not freeze the tartar sauce. Make a new batch of both sauces when you serve leftover fried chicken.

  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.