Make the marinade – Add the onion, sugar, mirin, and soy sauce into a bowl and stir together thoroughly.
Prepare the chicken – Cut the chicken into small, bite-size pieces, trimming off the excess fat. Add the chicken pieces into the bowl with the marinade and mix together thoroughly. Let marinade for 30 minutes or overnight.
Prepare the taco toppings – When you’re about ready to make the tacos and the meat has been fully marinated, prepare the taco toppings. Mince the onion and green onion into small pieces, slice the radish and pear, and set aside. Also, set aside the tortillas you are using at this point.
Cook the chicken – Heat a cast iron skillet or grill pan over the stove to medium-high heat. Place the chicken pieces in a single layer and cook for about 3-4 minutes on each side.
Toast the tortilla – Use a skillet set to medium to high heat to lightly toast the tortilla for about 30 seconds on each side.
Assemble the taco – Lay the tortilla out on a flat surface, then add the chicken, cheese, and toppings in that order. Serve and enjoy your chicken tacos!