Bring a large pot of salted water to a boil. Add the cauliflower and cook until very tender, about 10 minutes. Reserve ¼ cup of the cooking liquid, then drain well and transfer the cauliflower to a food processor. Add oil and reserved water, 1 tablespoon at a time, and purée until smooth. (Or, mash cauliflower with a potato masher.) Season with salt and pepper and serve.