Rinse and sort the rice for a minute in the colander with cold water. Pour the rice into a medium mixing bowl and cover it with 2 inches of cold water. Let the rice soak for 90 minutes. Drain the rice in a colander and rinse again with cold water, shake out the excess water. Rinse and dry the mixing bowl and set aside. Bring 3 cups of water to a boil in a 5 quart nonstick pot (using a pot with a nonstick surface is important; it will help in the formation of the crispy tahdig). Pour in the rice, beans, and salt. Stir. Bring the water back to a boil. Let the mixture boil uncovered on medium high for 8 minutes. Stir the rice periodically during cooking. While rice is boiling, crush the saffron threads in a mortar and pestle. Pour 1/2 cup of hot water into the mortar and let the saffron soak. After 8 minutes of boiling, pour the rice into a colander and rinse with lukewarm water, then drain. Rinse and dry the pot and put it back on the stove. Stir the fresh chopped dill into the rice in the colander, making sure it is well mixed throughout the rice. I use about 1 cup of chopped dill; you can use more if you like a very green rice with a strong dill flavor. In the medium mixing bowl, stir together 1/2 cup vegetable oil, half of the saffron water, and 1 1/2 cups of the cooked rice. Spread the oil and rice mixture in an even layer on the bottom of the pot. This will become your tahdig. Scoop the rest of the rice out of the colander and into the pot, forming a volcano-shaped mound in the middle of the pot. Make a hole in the center of the volcano with the handle of a wooden spoon, pushing it almost to the bottom of the pot. Place a thin kitchen towel over the top of the pot, being careful not to let the towel get too close to the heat source. Place the lid tightly over the top of the towel. Turn on the heat to medium and let the rice steam for 10 minutes. Meanwhile, mix together the remaining saffron water with remaining 1/4 cup of vegetable oil. After 10 minutes, remove the towel and lid from the pot and pour the saffron oil mixture evenly over the top of the rice. Replace the towel and lid. Turn heat to low. Let the mixture steam for 45-50 minutes longer. Move the pot to a cool surface and let it rest for 5 minutes. When ready to serve, you have two options. Option 1: Scoop the rice out of the pot and mound it onto a serving plate or tray, leaving a thin layer of rice undisturbed at the bottom of the pot. Use a spatula to loosen the crispy tahdeeg from the bottom of the pot and arrange the pieces around the cooked rice. Garnish with a little fresh dill, if desired. Option 2: Place a large serving plate upside down on top of the pot. Invert the pot quickly and gently unmold the rice from the pot. This will keep the tahdig intact and create a cake-like effect with the rice. Serve hot.