For preparing tans special chicken curry, wash the chicken pieces properly and keep it aside.
Heat oil in a pan, saute onions, garlic, ginger, green chillies and salt until the onions become light brown in colour. Remove it from flame and allow it to cool.
Grind the above into a fine paste with fenugreek seeds and mustard paste. Keep this fenugreek mixture aside.
Meanwhile cut the tomatoes and saute it in a pan with little water. Grind it to make a fine puree.