Cut the ends off the lemons so they sit flat, then cut off the peel, following the curve of the fruit. Cut each lemon lengthwise into quarters. Cut each quarter crosswise into small wedges, discard the seeds and set aside.
In a small saucepan, heat the olive oil and pistachios over low heat. Once the oil starts bubbling, stir and let sizzle for 30 seconds; turn off the heat. Stir the lemon pieces into the hot pistachio oil and set aside.
Cut off the ends of the zucchini, then cut each zucchini lengthwise into quarters, and then crosswise into small wedges, ¼ to ½ inch thick. Add to a medium bowl, season with salt, toss and set aside.
Season the pistachio-lemon oil with salt, black pepper and crushed red pepper, then pour it over the zucchini. Toss very well to combine, then serve in the bowl (or transfer to a platter) and, if using, sprinkle with flaky salt or feta (or both). The salad can be made and refrigerated up to two days in advance.
