Cook the onion in the butter with a good pinch of salt for about 20 minutes, until soft, dark golden brown and caramelised. Add the mushrooms to the pan and cook until softened and browned and there’s not much moisture left in the pan. Stir in the garlic and spices and cook for a minute then tip in the rice and stir well. Measure 2 cupfuls of stock using the same container you used to measure the rice, and add that to the pan. Cover with a tight-fitting lid and cook on a low simmer for about 12-15 minutes until the stock is absorbed and the rice tender.