Step 1 Rinse the rice thoroughly in 5 or 6 changes of water. Place in a large bowl, cover with water by 1 inch, stir in 3 tablespoons salt and set aside for 3 hours.
Step 2 Bring 2 quarts of water to a rapid boil in a 6-quart saucepot and add the remaining 3 tablespoons of salt. Drain the soaked rice and pour it into the boiling water. After 2 minutes, start testing the rice for doneness. When the grains are soft on the outside but still firm in the center, drain the rice in a colander and rinse it with tepid water. Toss gently in the colander to make sure the grains are separate.
Step 3Rinse the saucepan and put 6 tablespoons of the oil in it along with 2 to 3 tablespoons water. Heat the saucepot over high heat until the liquid sizzles. Using a large spoon, sprinkle the rice into the saucepot, building it up into a cone shape. Using the handle of a wooden spoon, poke two or three holes through the rice down to the bottom of the saucepan. Cover and cook over high heat until the rice is steaming, 2 to 3 minutes.
Step 4Wrap the lid of the saucepot with a clean dish towel and cover the pan firmly. Reduce the heat to low and cook at least 30 minutes. (The rice can be kept warm on the very lowest heat at least 1 hour longer.)
Step 5To make the garnish, heat the remaining oil in a frying pan over medium heat, add the minced onion and cook over medium heat until the onion is soft and golden, 5 to 6 minutes. Add the barberries, two-thirds of the saffron and the sugar and saute 2 to 3 minutes. Set aside.
Step 6Mix the remaining saffron with 2 teaspoons water. Remove the saucepan from the heat, set it on a cold, wet surface and let stand for a minute or two. This helps to release the crust of browned rice (tahdig) from the bottom of the pan. (The tahdig can be broken up and served separately as a treat.)
Step 7In a small bowl, combine 2 to 3 tablespoons of the rice with the saffron water to color it yellow.
Step 8To serve, use a large spoon to sprinkle about one-fourth of the rice onto a serving dish. Scatter one-third of the barberry-onion garnish on the rice, then continue alternating rice and garnish, ending with rice. Top with the grains of saffron-colored rice.