1. Remove leaves from inner stalks of celery; there should be about 1/2 cup. Refrigerate leaves to use later for garnish. Peel any tough strings from stalks and chop stalks into 1/2-inch pieces; there should be about 6 cups.
2. In a large saucepan, heat butter over medium heat. Add shallots and cook, stirring occasionally, until softened, about 4 minutes. Add chopped celery and potato, 1/4 tsp. celery seeds, thyme, bay leaves, and 1/2 tsp. salt, stirring to coat vegetables with butter and shallots. Reduce heat to medium-low, cover, and cook, stirring occasionally, until potato is tender and celery is tender-crisp and has released juices, about 15 minutes.
3. Add water and cook to blend flavors, about 1 minute. Remove bay leaves and thyme sprig, if used.
4. Working in batches, fill blender jar halfway with a portion of celery mixture and some of the parsley and purie, holding lid in place with folded dishtowel. Pour purie into a mesh strainer set over a large, non reactive bowl (preferably stainless steel). Gently stir purie in strainer until all liquid has passed through into bowl; press on solids to extract as much liquid as possible, then discard them. Once all the purie is strained, add remaining 1/2 tsp. salt and stir to combine. Let cool briefly to room temperature, about 20 minutes, cover with plastic wrap, and refrigerate until soup is cold and flavors have blended, at least 4 hours.
5. Correct seasoning with additional salt if desired. Ladle into chilled bowls. Mince reserved celery leaves and use to garnish each bowl, along with a few of the remaining 1/4 tsp. celery seeds and a thin slice of lemon.
Source: Hannaford fresh Magazine, July - August 2007