For the curry oil, place the curry powder in a dry frying pan and lightly toast for a few minutes until aromatic. In a separate frying pan, lightly roast the coriander seeds and cinnamon for a few minutes then stir in the curry powder. Pour in the vegetable oil, add the lime leaves and bring the temperature of the oil up to around 70C. Remove from the heat and leave to marinate in an airtight container in the fridge for 24 hours.
Preheat the oven to 180C/160C Fan/Gas 4. For the cauliflower steaks, cut two 2.5cm/1in thick slices from the centre of each cauliflower half. Use the remaining cauliflower in another recipe. Season each steak with salt and pepper. Heat 50ml/2fl oz of the curry oil in a frying pan and add the butter and thyme. Add the cauliflower steaks and cook for 5 minutes on each side. Transfer the steaks to a roasting pan and cook in the oven for 8–10 minutes, or until tender and golden brown.
For the salted grapes, mix the grapes with the oil and salt. Stir in the raisins and capers then season with some of the remaining curry oil to taste. Store any remaining curry oil in an airtight container in the fridge and use within a week.
For the polenta, bring the stock to the boil in saucepan and gradually add the polenta. Reduce the heat and simmer for 3–5 minutes. Remove from the heat and stir in the butter and Parmesan.
Place the steaks on warmed plates with the salted grapes and polenta alongside. Garnish with the mint and serve.