Heat the oil in a large pan over medium heat until it is hot.
Add the mustard seeds and cumin seeds to the hot oil and sauté them for about 1 minute until they begin to crackle and release their aroma.
Stir in the chopped onions and cook them for 4 to 5 minutes until they turn lightly browned and soft.
Mix in the garlic paste and sliced green chillies, then sauté for 1 to 2 minutes to blend the flavours.
Add the chopped tomatoes to the pan and cook for 4 to 5 minutes until they soften and break down.
Place the chicken pieces into the pan and cook for 4 to 5 minutes until the chicken changes colour and starts to cook through.
Sprinkle in all the spices and cook the mixture for 3 to 4 minutes to let the spices infuse the chicken.
Pour in the water, cover the pan with a lid, and simmer on medium heat for 15 minutes, stirring occasionally to prevent sticking.
After 15 minutes, add the achar, bullet chillies, and lemon juice, then cook uncovered for another 3 to 4 minutes to combine all the flavours.
Finally, garnish the dish with fresh coriander leaves, dried fenugreek, and finely chopped ginger before serving.
