Grind together whole black pepper, green chilli, ginger, garlic and coriander leaves with 2 tbsp water to a fine paste. Add this ground masala, lemon juice and salt to cleaned mutton pieces. Marinate it for half an hour to one hour.
Heat oil in the pressure cooker. Add sliced onions and cook till it becomes golden brown. Add all the masala powders. Mix well. Add a little water (1 tbsp) to mix everything well.
When oil starts appearing, add tomato, salt (keep in mind that salt is already added to mutton) and curry leaves. Mix well. After 3-4 mins, add marinated mutton pieces. Mix well.
Add 1 cup water, let it boil. Pressure cook till the mutton is done.
If there is any gravy, continue cooking, so that the water dries up and mutton is covered with masala. Garnish with freshly ground pepper and onion rings.
Serve it with rice, roti, pathiri etc;
Notes
This curry has a dry consistency. But I made a semi dry version. You can adjust the gravy to suit your taste. The more you roast on low flame, the more darker the mutton will be. It will be great if you can marinate mutton for 1-2 hours.