How to Make It
Drain the yogurt to thicken it: Line a sieve or colander with cheesecloth or a cotton cloth. Moisten the cloth with water. Set the sieve or colander over a bowl and add the yogurt. Set aside, loosely covered, to drain for about 30 minutes.
Meanwhile, trim the tough stems from the spinach. Wash the spinach thoroughly in several changes of water and drain well. Coarsely chop and set aside.
Heat the oil in a wide heavy skillet over medium-high heat. Add the onion, lower the heat to medium, and fry the onion until translucent and touched with color, about 5 minutes. Transfer the onion to a plate and set aside. Raise the heat under the skillet to medium-high and add the spinach, turning it to expose it to the hot surface. Add about 1⁄2 cup water and cook, pressing and turning the spinach, until it is well wilted and deep green, 4 to 5 minutes. Transfer the spinach to a bowl to cool slightly. Once the spinach is cool enough to handle, squeeze it thoroughly, a handful at a time, to press out excess water.
Transfer the spinach to a bowl, add 1⁄2 teaspoon salt, and mix well. Turn the thickened yogurt out into a bowl; you’ll have about 1 cup. Add the remaining 1⁄2 teaspoon salt and the water to loosen the yogurt slightly and stir. (Save the whey for another purpose or discard.) Add the yogurt to the spinach and stir gently to mix them a little, but not into a smooth blend, leaving the mixture with patches of white and dark green. Taste and adjust the seasoning if necessary. Strew on the fried onions, sprinkle with the saffron water and toasted walnuts if you wish, and serve.
You can make borani with other vegetables too. One of the most appealing is beet borani—the yogurt’s slight tartness is a great foil for the sweetness of the beets. Place 6 medium beets (1 1⁄2 to 2 pounds) in a roasting pan, coat with a little oil, and roast at 400 F until cooked through, about 1 hour, or boil them whole until cooked through. Let cool. Peel the beets and chop into about 1⁄2-inch dice. I have a weakness for beets with cumin or fennel, so I suggest tossing a generous pinch of one or the other into the pan as you fry the onion, along with 1⁄2 teaspoon salt. Once the onion is softened and touched with color, add it to the chopped beets. To serve, combine the beets with about 1 cup drained yogurt (from 1 1⁄2 cups full-fat yogurt) seasoned with 1⁄2 teaspoon salt, without mixing them completely. Top with about 3 tablespoons coarsely chopped toasted walnuts or, for a splashier look, with coarsely chopped pistachios.