Using very little water, grind all the ingredients for the masala into a smooth , thick paste.
Whizz the mutton mince in the blender or food processor along half the ginger and garlic, 1 green chilli, 6-7 curry leaves and a little salt till you have a soft, homogenous mixture.
Put the mince into a bowl and mix in 1 tsp of the masala paste. Combine well and shape into 8-10 meatballs. Chill for 30 mins in the fridge.
Heat coconut oil in a pan on low-medium heat. Tip in the sliced onions and half the curry leaves, Cook till the onions are translucent, then add the sliced ginger, green chillies and garlic and sauté for a min more.
Then add the masala paste and cook the paste for a couple of min till the oil leaves the paste. Then add the thin coconut milk, vinegar and salt. Bring to a simmer and gently add the meatballs. Cover and cook for 30-40 mins till the meatballs are tender and cooked through. Gently turn the meatballs halfway through cooking, using a teaspoon.
Now add the thick coconut milk, more salt if needed and the remaining curry leaves. Bring to a boil and turn off the heat.