Remove root, tough outer leaves, and tops from leek, leaving 1 inch of dark leaves. Cut into quarters lengthwise. Thinly slice leek; rinse well, and drain.
Cook leek and cauliflower florets in boiling water to cover in a large saucepan 10 minutes or until tender; drain. Return vegetables to saucepan, and toss in pan over high heat to remove any excess moisture. Remove from heat.
Process vegetables with 1/4 cup yogurt, in 2 batches, in a food processor until smooth, stopping to scrape down sides as needed. Stir in chopped parsley and next 3 ingredients; transfer puree to a serving bowl. Serve immediately.