Rinse the rice well - either under running water or in two or more changes of water until the water is clear. Put the rice in a pan and cover with plenty water and leave it to soak while you prepare the orange and carrots.
Cut strips of orange rind from the orange with a vegetable peeler and cut them into small strips (alternatively, use a zester if you have on but don't just grate as they will be too thin).
Place the orange zest in a small pan with boiling water and simmer around 5 minutes. Then drain the water, reserving the orange zest, and do the same again (simmer for 5min then drain).
Place the water and sugar in the same small pan and warm to bring it to a simmer. Add the orange rind and simmer gently for around 10 minutes until the syrup thickens slightly. Remove from the heat and take out the orange zest - set aside - but save the syrup.
Meanwhile, cut the carrots into matchsticks ( julienne) and warm around 1tbsp/15g butter in a skillet/frying pan. Add the carrots and saute a couple minutes until gently soft. Add the reserved orange zest and almonds to the skillet (and pistachios if using) along with around 1tsp of the orange syrup and cook a further minute or two.
Parboil the rice to al dente in the water it has been soaking in - you want it only just cooked so it is still firm but not hard in the middle. Drain the rice and set aside. Up to this point can be prepared ahead of time, if suits - see notes above in post on storing.
Warm the oil in a medium-large pot (can be the same one you boiled the rice in, just rinse out and dry) and add half the rice. Sprinkle on some saffron and add half the carrot, almond and orange mixture. Top with the remaining rice.
Top the rice with pieces of butter and make 4-5 holes for steam with the handle of a wooden spoon. Sprinkle on some of the leftover syrup from cooking the oranges - around 2tbsp but can adjust to taste. Cover the pan, putting a cloth over the pan first before the lid to absorb the steam, and leave to cook gently over a low heat for around 30-40 minutes.
If possible, flip the rice onto a plate so the the slightly crisp bottom sits on the top. Serve, topped with the remaining carrot and orange mixture.