Preheat oven to 180° C (350°F). On a baking sheet, heat pastry shells in oven for 10 minutes or according to package directions. In a saucepan, melt butter or margarine over medium high heat. Add celery and mushrooms. Cook until tender crisp, about 4 minutes. Blend in flour and stir for 1 to 2 minutes at low heat. Add milk and chicken broth and continue cooking, whisking until mixture thickens. Stir in eggs and ham. Season with pepper. Spoon over warm pastry shells. Sprinkle with poppy seeds.