Grind the black peppercorns, garlic, ginger and green chilli into a fine paste with a little bit of water. Add this to the washed mutton pieces; season with salt and add one sprig of curry leaves. Mix the masala well into the mutton and keep aside for at least 1 hour (longer if time allows).
Heat oil in a large deep bottom pan and add the sliced onions. Sauté till the onion are caramelized to a light brown colour.
Next add the tomatoes and continue to sauté till the tomatoes are completely broken down and mushy.
Then add all the spice powders and mix well to combine. Sauté till the whole mixture comes together and oil starts appearing at the sides. A few drops of water can be added if the mixture feels too dry.
Add the marinated mutton to this along with one sprig of curry leaves; mix well to combine. Add 2 cups water (taste and season with salt if necessary) and cook till the mutton is almost done. (You can also use a pressure cooker for cooking the mutton but add less water).
When the mutton is almost done, increase heat and reduce the excess gravy if any to get a thick masala like consistency. But if you prefer the gravy, remove from heat and garnish with the remaining curry leaves.