Achari (sometimes known as achaar or achar) means pickle. Basically, this is a chicken dish that is made with all the spices one would normally use in an indian pickle. It is extremely aromatic and delicious, and as is the case with most pickled foods, the flavour improves over time. So this is the perfect 'make-ahead' dish that will taste even better a day or two after it was prepared. It goes well with plain rice or chapatis/parathas.
Marinate the chicken cubes in the chicken marinade ingredients for at least 30 minutes or up to overnight.
Mix the cumin, coriander, fennel and split fenugreek seeds, roast them in a dry pan for a minute or two until they are aromatic, then crush them to a rough powder.
Drizzle some of the oil in a pan, when hot fry the chicken pieces in batches over high heat just to seal the chicken and keep it moist. Do not be tempted to over crowd the pan as that will drop the temperature of the oil and the chicken will release a lot of moisture. Keep the heat high and stir fry the chicken in two batches for about 3 minutes per batch, then remove the cubes into a bowl, cover and set aside. Bear in mind that the chicken will be cooked further in the gravy.
In the same pan, add the rest of the oil then add the mustard seeds. When they begin to splutter, add the chopped limro/curry leaves followed by the chopped onions. Fry on medium heat until the onions are golden. Then add the ginger and garlic pastes. Stir them in for a few seconds, then add the tomatoes and tomato paste, crushed roasted spices, salt and red chilli powder. Cook until the tomatoes are thick and oil begins to get released from the sides. Then whip the yoghurt and add it in, give it a mix to combine.
Add the chicken pieces and the slit green chillies, give them a stir then cover and turn down the heat. Simmer for about 15 minutes on low heat or until the chicken pieces are tender and cooked. Squeeze in the lemon juice and add in the tablespoon of achaar (if you have it). This will enhance the flavour of the chicken even further but you can leave it out if you do not have pickle. Stir and taste, adjust seasoning if needed. Turn off the heat and allow the dish to sit for about 15 minutes before serving to ensure all the flavours are infused into the chicken. Serve with rice or parathas/chapatis.