Prepare the achari spice powder by adding red chilies, coriander seeds, cumin, mustard, fennel seeds and methi seeds to a spice jar.
Make a fine powder set aside.
Beat the yogurt well with a whisk until smooth.
Heat oil in a pan. If using mustard oil, heat it until smoky hot.
Add kalonji seeds & mustard. When they splutter add hing.
Next add onions and saute until golden stirring often.
Add ginger garlic paste and fry till the raw smell goes away. Reduce the flame and stir in the achari spice powder.
Saute until a nice aroma comes out just for 1 to 2 mins. Avoid over frying as it can turn bitter.
Pour the tomato puree and saute till the masala thickens. Then add red chili powder, turmeric, salt and sugar.
Saute on a low heat for 2 to 3 mins until the masala leaves the pan. Take a tablespoon of this masala and mix it with the whisked yogurt.
Ensure flame is very low at this stage and add half of the yogurt. Stir well to incorporate it with the onion tomato masala.
Add the rest of the yogurt and mix well. Cook until the masala leaves the pan.
Transfer the chicken and saute it for about 3 to 4 mins on a low heat.
When the chicken turns pale, cover and cook for about 3 to 5 mins stirring occasionally.
By now chicken releases some moisture, based on this add some hot water to make a thick masala. You may need anywhere in between ½ to 1 cup water depending on the chicken. If you prefer your curry to be on the thicker side & not runny you will need only half cup.
Cover and cook until the chicken is tender & cooked fully. If using achaar or pickle, stir it at this stage. Taste it and add more salt or sugar at this stage.
Add ginger juliennes. Cover & rest the achari chicken for at least 30 mins before serving.
If desired squeeze in some lemon juice before serving. Serve with basmati rice, roti or naan.