In a large pot (I use a Dutch oven), heat 1 Tbsp neutral oil on medium-high heat. When it’s hot, sear the chicken on the skin side first. Do not crowd the pan to avoid steaming the chicken and cook in batches if necessary.
Flip the chicken and cook the other side for 2 minutes and then transfer to a plate until the rest is done.
Add the onion to the pot and coat with the oil using a wooden spoon/spatula. If there is not enough oil, add ½–1 Tbsp more oil.
Add ginger slices and sauté, stirring occasionally, until the onions are tender. If the burnt spots on the bottom of the pot need some scraping, you can add sake here (instead of the next step) to loosen them up.
Add the chicken back into the pot and add ¼ cup sake.
Add 2 cups chicken stock/broth and ½ cup water, or water plus chicken powder just enough to almost cover the chicken. If you use a larger pot, it may not cover, so please adjust the amount of stock/water. This is not a soupy curry, so we do not need much liquid.
Cover the lid and bring it to a simmer. When simmering, skim the scum and foam with a fine-mesh skimmer.
Cook covered on low heat for 40 minutes.
Add the carrots, potatoes, and any other veggies you like, make sure they are submerged in the cooking liquid. Options include bell peppers and cauliflower or squash
Cook covered until they are tender, about 15 minutes.
Turn off the heat and dissolve ½ package Japanese curry roux, one cube at a time in a ladleful of hot broth. When the first cube is dissolved, release it to the broth and mix gently without breaking the tender vegetables. Repeat the process until you finish dissolving all the roux.
Add 1 Tbsp mirin and 1 Tbsp soy sauce. Mix all together and check the taste. You can add water to loosen the curry if it's too thick. Add 2 Tbsp water. Add more water, if necessary to control the consistency of the curry here. The sauce should be pretty thick though.
Add ¼ cup green peas and cook for another minute. Serve the curry with steamed rice.