heat 2 tbsp oil in a large saucepan over med-high heat. Add 1 tbsp of butter.
add dry whole spices, cloves, cinnamon, cardamom, fennel seed and curry leaves. Fry them for about a minute until it is fragrant.
Add crushed ginger and garlic, saute for 2-3 minutes, stir frequently, until the raw smell is gone.
Add sliced green chillies and sliced onions. Saute them until the onion is light brown. Onion does not need to be golden brown for stew.
Add marnated chicken pieces. Add 1/2 of the coconut milk, black pepper and salt as needed. Mix it all very well and cook covered over medium heat for about 5-10 minutes.
Once the chicken is cooked, add cooked potatoes and carrot to the chicken. Combine everything well. Cover and cook for another 5 mintues.
Then add the remaining coconut milk, Vinegar, 1/2 tsp black pepper, and 1/2 tsp garam masala. Let it boil for a few minutes until you get the desired consistency.
In a small frying pan, heat oil and then add mustard seed, whole chilli and curry leaves and allow it to splutter. Pour that over the chicken stew.
Turn off heat and enjoy!!!