Heat oil in a pressure cooker. Add cinnamon, cloves, cardamom, black pepper and fennel seeds. Fry for 2 mins. Add sliced onion. Cook till it becomes brown. Add crushed ginger garlic and green chilli. Cook for 2-3 mins, till the raw smell goes.
Add coriander and turmeric powder. Mix well and cook for 3-4 mins. Add poppy seeds (if using) and cook for 2-3 mins. Remove the masala from the gas and let it cool completely. Grind the cooled masala with 1/4 - 1/3 cup water to a smooth paste.
Grind the soaked cashews to a smooth paste and keep aside.
Add salt, vinegar, ground paste and 3/4 - 1 cup water to cleaned mutton and mix well. Pressure cook mutton till it's done (refer notes). Once the pressure drops, open the cooker and mix well. Add cashew paste to the mutton and cook for 3-4 mins. Add thick coconut milk and mix well. Cook on low flame for 6-8 mins. Add chopped coriander leaves and mix well.
Let the curry rest for 20-30 mins, for the flavour to settle in.
Serve with plain rice, ghee rice, veg pulao, tomato rice, appam, parotta, idiyappam etc;