Heat the butter in a pan over a medium heat, then add the onion and sauté for 8–10 minutes, until it turns completely soft. Add the spices, cherries and a generous pinch of salt and cook for 1 minute more.
Add the rice and pistachios and give a good stir over the heat for a minute or 2 to toast the rice, then add the liquid and check the seasoning.
Put a lid on the pan and cook the rice on high for 3 minutes, then turn the heat down to low and continue to cook for 15 more minutes, until all the liquid has evaporated and the rice is tender.
Take off the heat. Place a clean tea towel between pan and lid and let the pan sit and rest for 5 minutes. The tea towel will help draw excess moisture from the pan and helps to keep the rice extra fluffy.
While the rice is cooking, season the yoghurt with a bit of salt, the garlic and the sumac, if using.
When the rice is ready to serve, with a fork stir through the majority of the herbs, fluffing the rice as you go.Serve the pilaf at the table, with the yoghurt, chilli flakes, remaining herbs and lemons to add per bowlful.