To a cooking pot(I use 12 inch stainless steel indian style kadai(wok)), add the oil and let warm up.
Add potatoes to the hot oil. Sprinkle ¼ teaspoon salt and lightly brown the potatoes on all sides without cooking them through. Using a slotted spoon, take out the potatoes in a bowl and set aside.
To the remaining hot oil, add the chopped onions. On low medium heat, brown the onions until lightly brown. Takes about 5-6 minutes. Don't brown too much else the taste of onions gets overpowering.
To the browned onions, add the vindaloo paste that we saved and kashmiri red chilli powder. Fry on medium heat for 1-2 minutes.
Layer the marinated chicken in the pot. Add the fresh curry leaves. Turn the heat to medium high and let the chicken brown. In about 3-5 minutes, flip the chicken pieces and let brown on the other side. You will slowly see chicken juices releasing.
Next go in the shallow fried potatoes, tomato sauce and about ½ cup water. Please keep in mind that the chicken will release its juices once we cover. You can add more water at any stage.
Cover the pot and let slow cook for low heat for 15-18 minutes or until the chicken is cooked through and potatoes are tender(but not mushy).
Uncover, add the jaggery(or sugar), taste & adjust the salt. Another thing to keep in mind is that vinegar will cook off, so if you feel that vindaloo is less tangy, add a splash of vinegar to bump the tangy taste. At this stage, you can add more water to adjust the consistency as per your wish. Let simmer uncovered or another 5 minutes.
Let sit for 30 minutes before serving.Serve warm with steamed basmati rice.