What and where is mutton nalli!

I have a friend visiting Trivandrum for a day, and wants to have mutton nalli. I'm not much of a explorer myself so help me out please! 1. What is mutton nalli? 2. What are the best places for it? 3. Is there a specific time that it is available, atho eppo poyaalum kittuo? Thanks in advance dear people! You're also free to suggest any other exceptionally good stuff in tvm(other than mother's plaza sadhya)