In a measuring cup or small bowl, combine 6 Tbsp ketchup, 4 Tbsp mirin, 4 Tbsp soy sauce, 2Tbsp rice vinegar, 2 Tbsp sugar, 1 Tbsp sake, and 1 tsp potato starch and whisk well.
To Make Meatballs
Mince the onion into fine pieces.
In a bowl, combine ground chicken, minced onion, 1 large egg, 3 Tbsp panko, 1 Tbsp potato/corn starch, 1 Tbsp sake, ½ tsp kosher salt and freshly ground black pepper. Knead the mixture well with your hand until the mixture is pale in color and very sticky.
Apply a small amount of cooking oil to your hands and make a ping-pong ball size shape with the mixture. If you have a cookie scooper, it helps to make equal size balls.
Once you make a ball, place it on a tray or plate lined with a sheet of parchment paper. Once you finish shaping, cover the tray/plate with plastic wrap. Refrigerate for 15 minutes so the melted fat on meatballs will solidify.
Put ½ inch (roughly 1 cm) water in a large frying pan and bring it to boil. Carefully place the meatballs in the water.
Cover and cook on medium heat for 5 minutes. After 5 minutes, remove the lid and let the water evaporate while you continue cooking the meatballs on medium high heat, turning the meatballs once in a while.
As water evaporates, the meatballs will turn brown. If the frying pan gets a burnt spot, wipe off with paper towel.
Add the sweet and sour sauce and coat the meatballs well with the sauce. Turn off the heat and serve.