Marinate the chicken pieces with half the qty of onion, ginger & garlic (half the qty), green chili (1/2 of the total qty used), coriander powder (1/2 of the total qty used), garam masala (half of the total qty), turmeric powder, chicken masala, curry leaves & salt. Keep it aside for 10 minutes. Cook the marinated chicken in medium thick coconut milk, till the chicken becomes tender.
Heat oil in a pan & splutter mustard seeds. Add small onion, remaining portions of the sliced big onion, ginger & garlic, green chili 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.