In a medium enameled cast-iron casserole, melt the butter. Add the onion, carrot, garlic, jalapeño and ginger and cook over moderately high heat until softened, about 5 minutes. Add the curry leaves, Madras curry powder, turmeric and fenugreek and cook until fragrant, about 1 minute. Add the fish stock, coconut milk and tomato and season lightly with salt. Cook over moderately high heat, stirring occasionally, until slightly reduced, about 15 minutes. Add the mussels and shrimp and cook until the shrimp are opaque and the mussels have opened, about 3 minutes; discard any mussels that don't open. Stir in the cilantro and lemon juice and serve with steamed rice and naan.