Set up a collapsible steamer or bamboo steamer over a pot of water. Toss the cauliflower, potato, and zest together and put in the steamer. Season with the salt and pepper, to taste. Cover, and cook over medium heat until the potatoes and cauliflower are completely tender, about 15 minutes. Cool slightly.
Puree the vegetables, in batches if needed, in a blender or food processor with olive oil and a couple tablespoons water, until smooth and creamy. Season with the salt and pepper, if needed. Reheat in a saucepan, stirring over medium-low heat. Serve.
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