Pulse cauliflower in four batches in a food processor until finely chopped (to create pieces about the size of couscous). Set aside.
Heat oil in a 12-in. frying pan over medium heat. Add onion and garlic and cook, stirring occasionally, until softened and starting to brown, about 10 minutes. Stir in turmeric, salt, and lemon zest and cook 30 seconds. Add cauliflower and cook, stirring often, until softened, 8 to 10 minutes.
Transfer to a serving bowl and sprinkle with cilantro.