Pour the oil into a large roasting tin or ovenproof dish and put in the oven for a couple of minutes to heat through. Add all the vegetables to the tin, except the beans, stirring to coat them in the hot oil. Season with salt and pepper and return to the oven for 15 minutes until biryani to brown. While the vegetables are roasting, stir together the stock, curry paste, chilli, saffron and mustard seeds.