Toast coriander seeds in a dry, medium skillet over medium-low heat until they begin to turn brown and pop; transfer to the bowl of a food processor or a spice grinder. Toast dried chile peppers and cumin seeds, separately, in the same skillet, transferring to the food processor once each toasted. Add salt to the food processor; grind to a fine powder. Combine spice mixture, garlic paste, and ginger paste to form a thick curry paste.

Toast fennel seeds in the same skillet; set aside. Sprinkle turmeric over lamb until lightly coated.

Heat a Dutch oven over medium heat; add ghee and vegetable oil. Add onions; cook until golden brown, about 10 minutes. Stir in curry paste; cook for 1 minute. Add lamb; cook for 1 minute. Add 1 cup water and 2/3 can coconut milk; bring to a boil, then reduce heat to low. Simmer for 10 minutes.

Stir in remaining 1 cup water, remaining 1/3 can coconut milk, curry leaves, cardamom pods, cinnamon stick, and fennel seeds. Cover Dutch oven with lid slightly ajar; return to a simmer. Cook until lamb is tender, about 1 1/2 hours, stirring occasionally and thinning with water if sauce becomes too thick.

Stir garam masala and sugar into lamb mixture. Stir 3 tablespoons warm water and tamarind paste together; add to lamb mixture. Cook until sauce thickens, about 5 minutes more. Remove and discard cardamom pods and cinnamon stick before serving.