For the chicken: Place the ice cubes in a small bowl, sprinkle the crushed saffron over the top and set aside. As the ice melts, the saffron will slowly release its bright orange colour and pungent flavour. Melt the butter in a sauté pan on a medium heat and fry the chicken pieces for 5 minutes on each side, until lightly golden. Add the onion quarters, sprinkle with the turmeric, salt and pepper and cover with the water. Bring to the boil and skim off any foam on the surface. Lower the heat, cover and simmer for 15 minutes. Turn the thighs and simmer, covered, for another 15 minutes. Meanwhile, in a separate pan, fry the sliced onion in the ghee or vegetable oil for 10–15 minutes, until golden brown. Add the tomato purée and stir for a minute to coat the onion. Set aside. Remove about half of the stock and the onion quarters from the chicken pan. Reserve the stock for later (save the onions for a soup or a stew). Pour the saffron water over the chicken. Add the fried onion and tomato purée mixture, the lemon juice and barberries and stir well. Cover and simmer for another 15 minutes. (This is a good point to start making the barberry rice, see below.) Turn the chicken thighs again and simmer, covered, for a final 15 minutes. The sauce should be well reduced and glossy. If the sauce is too thin, remove the chicken and boil to reduce until thick; if it is too thick, add some of the reserved stock. Keep warm while you finish the rice. For the barberry rice: In a large heavy-based pan (covered with a tight-fitting lid), bring the water to the boil. Put the rice in a bowl and wash several times under running water, stirring gently with your fingers. When the water is clear, drain the rice. Mix the cinnamon, rose petals, cumin and cardamom until well blended. Set aside. Add the salt to the boiling water, then add the rice. Let the water come back to the boil, stir the rice and cook, uncovered, for about 7 minutes. It should be soft on the outside and slightly hard inside. Drain the rice. Return the pan to the heat and add 3 tablespoons of the ghee or vegetable oil. Spoon half of the rice into the pan, then sprinkle with half of the spice mixture. Spoon the remaining rice on top and sprinkle with the remaining spices. Poke the handle of the spoon into the rice, making five holes to let the steam escape. Wrap the lid in a clean tea towel and place on top of the pan. Turn the heat to low and cook for 30 minutes. For the rice garnish, heat the remaining 2 tablespoons of ghee or vegetable oil in a small frying pan on a medium heat and fry the onion for 10 minutes, until golden. Drain the barberries and squeeze out the excess water. Add them to the onion and stir for 30 seconds, taking care as the barberries tend to burn easily. When ready to serve, stir the rice gently with a fork to mix the spices evenly, and spoon onto a serving plate. Garnish with the onion and barberry mixture and sprinkle chopped almonds and/or pistachios over the top. Serve with the chicken.