Mix soy sauce, mirin, garlic, ginger, white pepper powder, scallion, onion and chicken thigh together in a big bowl, let it sit for between 10 minutes to a few hours few hours (overnight is ok) in the fridge.
Heat a frying pan over medium to medium high heat, pour the chicken with everything in the bowl into the frying pan, spreading it out into an even layer.
Cover and cook for 3 ~ 5 minutes until the surface of the chicken turns white.
Remove the lid and add in mushrooms, stir and cook until the chicken is cooked through and the liquid in the pan is reduced to just cover the bottom of the pan.
Mix starch with 2 tablespoon of water, pour the mixture into the pan and stir for several times until the starch mixture is evenly distributed and the liquid thickens up a little bit.
Remove the pan from the heat and season with salt to taste.
Serve with warm rice, top with fried egg or poached egg and enjoy!