For the brisket, preheat oven to 150°C/300°F.
For the avocado raita, combine all ingredients in a food processor. Refrigerate until ready to serve.
To serve, lightly toast the naans on a barbecue or in a frying pan. Serve filled with brisket, lettuce, tomato, cucumber, avocado raita and top with a squeeze of lime, some spring onions and red chillies. Finish with mango chutney and wedges of limes, if desired.