1. Preheat oven to 400°. Melt butter over medium-high heat in a large Dutch oven. Sauté potato, leeks, and celery for 10 minutes. Add thyme and garlic; cook 1 minute. Sprinkle flour over vegetables, and cook, stirring constantly, 3 minutes. Whisk in milk; bring to a boil. Reduce heat to low, and simmer 2 to 3 minutes or until potato is almost tender. Add cod and next 3 ingredients; return mixture to a simmer. Remove from heat. (Fish will be undercooked.) Ladle filling into 4 to 6 individual ovenproof soup crocks.
2. Roll out pastry on a lightly floured surface until smooth. Cut pastry into circles, cutting the circles 1 inch larger than the mouth of the soup crocks. Whisk together egg yolk and 1 tablespoon water; brush underside of pastry edges with egg wash (see glossary, page 140) and place over crock, pressing edges to seal. Brush tops with egg wash. Place crocks on baking sheet. Bake at 400° for 14 to 16 minutes or until pastry is golden brown.