In a microwavable bowl, combine mirin, tamari (soy sauce), sugar. Microwave for 30 seconds to blend the sugar into sauce.
Using a meat tenderizer, flatten chicken thighs into an even thickness. Sprinkle chicken with salt and pepper on both sides.
Heat a large flat-iron skillet (or large saute pan) to medium-high heat --make sure it's thoroughly heated. Add oil, then saute the chicken for 7-9 minutes each on both sides. After 5-6 minutes on the second side (when the chicken starts to cook), chop the chicken into bite size chunks then stir for about a minute.
Deglaze with a generous amount of sauce that covers all the pieces (reserve some as extra sauce) and stir for about a minute.
Transfer to serving dish and add extra sauce (optional: just before adding the extra sauce, caramelize and thicken extra sauce on a saute pan by cooking it on high heat until it darkens.).