Marinate the chicken pieces with onion(1 1/2), ginger & garlic (1 tbsp each), green chilly(1/2 qty used), coriander powder(1/2 qty used), garam masala(1 tsp), turmeric powder, curry leaves & salt.
Keep it aside for 10 to 20 minutes. Cook the marinated chicken in medium thick coconut milk, till the chicken becomes tender(pressure cooker is not recommended).
Heat oil in a pan & splutter mustard seeds. Add small onion, remaining portions of the sliced big onion, ginger & garlic, green chillies and curry leaves. Saute it till the onion becomes soft.
Add remaining coriander powder & garam masala. Fry it till the oil starts appearing. Add the sliced tomatoes. When tomato becomes soft add cooked chicken along with the gravy. Combine well.
Add thick coconut milk and cook for 5-7 minutes. Chicken mappas is ready.