Combine first 4 ingredients in a large Dutch oven; bring to a boil. Reduce heat; cover and simmer 2 hours. Remove chicken from broth; let cool slightly. Remove meat from bones, discarding bones; chop meat into bite-size pieces. Strain broth, reserving 1 quart. Discard quartered onion and celery. Reserve remaining broth for use in other recipes.
Sauté mushrooms and chopped onion in butter in a large Dutch oven until tender. Stir in flour. Cook over medium heat 5 minutes, stirring constantly. Gradually add reserved 1 quart chicken broth; cook over medium heat, stirring constantly, until thickened and bubbly. Stir in reserved chicken, pimiento, salt, and pepper. Remove from heat, and keep warm.
Cut a slice from top of each Kaiser roll to form a lid; set aside. Remove soft center from each roll, and reserve for other uses, leaving shells intact. Place rolls and lids on baking sheets. Bake at 350° for 8 minutes or until lightly browned and crisp.
Spoon warm chicken mixture into rolls. Place each filled roll and lid on an individual serving plate, and serve immediately.
Oxmoor House Homestyle Recipes