If using desiccated coconut, soak in a bowl of warm water to cover 1 hour and drain well in a sieve.
Purée coconut, garlic, and ginger with 1/2 cup water in a blender, then blend in remaining 1/2 cup water, fennel, coriander, chile powder, and turmeric.
Heat oil in a 6- to 7-quart heavy pot over moderately high heat until hot but not smoking, then cook cumin, fenugreek, fennel, cardamom pods, and cinnamon stick, stirring, until fragrant, about 30 seconds. Add curry leaves and cook, stirring, until fragrant, about 30 seconds. Add onion and cook, stirring, until softened and begins to brown. Add tomatoes and cook, stirring occasionally, until softened, about 1 minute.
Add coconut spice paste and salt and cook, stirring occasionally, 5 minutes. Add lamb and cook, stirring occasionally, until no longer pink on outside, 2 to 3 minutes. Add water and simmer, covered, stirring occasionally, until lamb is very tender, about 1 1/2 hours. Transfer lamb with a slotted spoon to a bowl and simmer sauce until thickened. Return lamb to pot and season with salt.
