For the soup: In a large saucepan, melt butter over medium heat and add onion, leek, celery, thyme, garlic, bacon, and bay leaf. Sweat the mixture over medium-low heat without allowing it to color at all, stirring occasionally, for 5 or 6 minutes or until very tender. Add potatoes, stock, and cream. Liquid should cover potatoes; if it does not, add a little water. Bring mixture to a simmer and cook, partially covered, for about 20 minutes, until potatoes are tender. Puree mixture in a blender and add creme fraiche. Push the soup through a fine strainer into a clean saucepan and season, to taste, with salt and white pepper. Cover and set aside.
For the Knish: Preheat oven to 400 degrees F. Cut puff pastry sheet into 4 (3-inch) circles and 4 (4-inch) circles. In medium saucepan, combine potato with milk and water and bring to a simmer over medium high heat. Simmer, partially covered, for about 20 minutes or until potato is tender. Drain.
Meanwhile, in small saute pan, heat 1 tablespoon butter over medium heat and add onion. Saute 3 to 4 minutes, until softened, then remove from heat and stir in parsley. Put potato through a food mill or mash with a masher or hand blender and stir in butter and parsley mixture. Season, to taste, with salt and pepper.
Rinse a large baking sheet with cold water. Set the 4 smaller circles of puff pastry on baking sheet and mound about 2 tablespoons of mashed potato in center of each circle, leaving edges clear for attaching pastry tops. Brush edges of circles with a little beaten egg. Score each of the 4 larger circles with the back of a knife in a lattice pattern, and using a 1/4-inch aspic or biscuit cutter, cut a small circular vent hole in the center. Form these circles into domes with hands and fit over the mashed potatoes, pressing down firmly around egg-washed edges to form a good seal. Brush knishes with remaining beaten egg and bake for 10 to 12 minutes, or until puffed and golden.
Assembly: Place a knish in each of 4 heated soup bowls. Reheat soup gently, if necessary, and ladle some hot soup into each bowl around the knish. Or if you prefer, you can serve the knish on the side.
Recipe courtesy Joachim Splichal, "The Patina Cookbook"