Heat the oven to 200C/180C fan/gas 6. Toss the beetroot with half the oil and some seasoning, then tip into a roasting dish and cook for 25-30 mins, tossing halfway through, until tender.
Meanwhile, heat the remaining oil in a large, shallow casserole dish or ovenproof frying pan. Fry the onion over a medium heat for 10 mins until golden. Add the ginger and half the garlic, and cook for 1 min. Stir through the bay, cardamom pods, turmeric and garam masala, then cook for 2 mins. Stir in the rice and beetroot, then season. Pour in the stock and bring to the boil.
Put in the oven, uncovered, and cook for 20-25 mins until the rice is cooked through. Give it a stir when it’s out of the oven.
Put the yogurt, coriander and remaining garlic in a food processor and whizz until smooth. Season to taste. Serve the biriyani with the coriander yogurt and pickle.