Pulse the cauliflower in a food processor until the pieces are about the size of rice. You'll probably have to do multiple batches. (You could also use a grater to do it by hand if you prefer.)

Heat the olive oil over medium heat in a large skillet. Add onion and cook until translucent and softened. Add the riced cauliflower and coat it evenly with the oil. Add the salt, pepper, and lime zest. Cook, stirring frequently, about 5 minutes or until the cauliflower just begins to soften.

Remove from heat and stir in cilantro.