Bring 2 cups water to a boil. Add the rice and 1/2 teaspoon salt, stir to even out the grains, and bring the water back to a boil. Turn the heat to low, cover, and cook until the liquid is absorbed, about 15 minutes.
Meanwhile, cut off the stems from the spinach. Plunge the leaves into plenty of cold water and wash well, twice if need be, then dry.
Heat the oil with the garlic over medium-high heat until the garlic begins to turn pale gold and flavor the oil. Discard the garlic, then add the spinach and a couple pinches of salt. Cook over high heat until wilted, which will happen rather quickly, then turn off the heat. When cool enough to handle, chop the spinach, then put it in a bowl and add the lemon zest and dill.
Turn the rice into the bowl with the spinach and toss well. Taste for salt. Add the feta and pistachips and toss again. Season with black pepper and with a few pinches of red pepper flakes. Serve immediately or let cool a bit.