Heat your oven to 220°C/Fan 200°C/Gas 7. Tip the rice into a bowl. Cover with cold water. Whisk (you can use a fork) for 1-2 mins till the water turns cloudy (this is the starch). Drain the rice and tip back into the bowl. Cover with more cold water. Set aside.
Add 1 tbsp oil to a shallow ovenproof casserole dish or roasting tin. Slide into the oven to heat. Pull the leaves off the cauliflower and break it into small florets, chopping any stems into bite-size pieces. Peel and finely slice the onion.
Add the cauliflower and onion to the casserole dish. Toss to coat in the oil. Season with a little salt and pepper. Roast in the oven for 20 mins.
While the veg roast, peel and grate the ginger. Finely chop the chilli, flicking out the seeds and white bits for less chilli heat. Crumble the stock cube into a heatproof jug. Add the chilli and ginger.
Bash 3 of the cardamom pods with a rolling pin or pestle to just open them. Add to the jug with 1 bay leaf, 1 cinnamon stick, 6 cloves, 1 tsp turmeric and 1 tsp garam masala. Stir 450ml boiling water into the jug. Turn the oven down to 180°C/Fan 160°C/Gas 4.
Drain the rice. Take the dish out of the oven. Stir in the rice. Pour in the vegetable stock, knocking in any whole spices. Season. Cover with a lid (or foil). Bake in the oven for 25 mins till the stock is absorbed.
While the rice cooks, roughly chop the cashews. Fish the bay leaf, cinnamon stick, cloves and cardamom pods out of the rice. Taste and add more salt and pepper if you think it needs it. Gently stir in the spinach to just wilt it. Serve the biryani in warm bowls topped with the chopped cashews.