Sabzi polo is a staple on the Persian New Year’s table. It’s made with basmati rice and layers of fresh herbs, but what is most coveted is the tahdig—the crunchy crust that forms on the bottom of the pot during cooking. Slideshow: More Rice Recipes
In a large bowl, cover the rice with cold water. Using your hand, agitate the rice to release the starch, then carefully pour off the water. Rinse the rice 5 to 7 more times, until the water runs clear. Cover the rice with water, add 2 teaspoons of salt and let soak for 30 minutes, then drain.
Fill a 5-quart nonstick Dutch oven half full with water, bring to a boil and add 2 tablespoons of salt. Add the rice and boil over high heat until it is al dente and just starts to soften; this can take from 3 to 8 minutes, depending on your rice (do not overcook it). Let stand for 2 minutes, then drain well. Wipe out the pot.
In a medium bowl, mix the cilantro, parsley, chives and dill with the garlic. Put the oil in the bottom of the Dutch oven. In a small bowl, whisk the yogurt with the egg yolk and 2 tablespoons of the brewed saffron and drizzle over the oil in the pot. Gently scatter one-quarter of the rice into the pot (you want it to stay fluffy) and season with salt. Sprinkle with one-third of the herb mixture. Repeat the layering with the remaining rice and herb mixture, ending with a layer of rice.
Cover the pot and set it over moderately high heat until it starts to steam, about 5 minutes. Uncover the rice and drizzle with the remaining 2 tablespoons of brewed saffron and 1/4 cup of boiling water. Scatter the butter slices on top. Cover the pot with parchment paper, then cover tightly with the lid. Cook on a heat diffuser (if you have one) over moderately low heat until the rice is tender and very fragrant, about 45 minutes.
Discard the parchment. Carefully invert the rice onto a large platter. Using a spoon or spatula, crack the crust (tahdig) and spread the pieces apart. Drizzle the rice with a little brewed saffron and serve warm.
Why don't you say "Iranian" instead of "Persian"?