Vegetable biryani

Name: Vegetable biryani
Ingredients: 2 tbsp vegetable oil; 1small cauliflower, broken into small florets; 2large sweet potatoes, peeled and cubed; 1large 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 seed (black or white); 500g basmati rice; 140g trimmed green bean, halved; 2 lemons, juice only; a handful of fresh coriander leaves; 50g packet salted roasted cashew nuts; poppadums and raita, to serve (optional)