Combine the garam masala, lemon zest, chilli powder, salt, pepper and mint and mix together to prepare the marinade. Add the yoghurt, taste test and adjust salt if necessary. Coat the olives in the marinade and transfer to an ovenproof baking dish.
Bake in the oven (or tandoor oven at a high temperature) for 6 to 8 minutes until the olives are tender. Remove from the oven and add a little lemon juice to taste; garnish with chopped fresh coriander and a pinch of tandoori powder and serve.