Spiced vegetable biryani

INGREDIENTS

• 2 tbsp vegetable oil
• 1 small cauliflower , broken into small florets
• 2 large sweet potatoes , peeled and cubed
• 1 large onion , sliced
• 1l hot vegetable stock
• 3 tbsp hot curry paste (Madras is good)
• 1 red chilli , seeded and finely chopped
• large pinch of saffron strands
• 2 tsp mustard seeds (black or white)
• 500g basmati rice
• 140g trimmed green beans , halved
• 2 lemons , juice only
• a handful of fresh coriander leaves
• 50g packet salted roasted cashew nuts
• poppadoms and raita, to serve