Pressure cook mutton (refer notes) with crushed ginger & garlic (1 tbsp, each), green chilli (3), cubed potato, salt, pepper powder and 2 cups water, till mutton is cooked.
Heat oil in a deep and wide pan and add the crushed whole masala (cardamom, cloves, cinnamon, fennel seeds and peppercorns). Fry for a min. Add sliced onion and cook till it begins to brown (no need to brown).
Add crushed ginger & garlic (1/2 tbsp, each), green chilli (2) and curry leaves. Cook for 2-3 mins, till the raw smell goes. Add the cooked mutton along with the gravy. Mix well. Bring it to boil, simmer and cook for 5-7 mins, till the gravy reduces a bit. Add medium thick coconut milk and mix well. Bring to boil and simmer for 7-8 mins. Add thick coconut milk and mix well. Cook for 5-7 mins, till the gravy becomes thick. Let the stew rest for half an hour for the flavour to settle in.
Serve with Appa, Idiyappam, Puttu, Roti or Bread