Combine celery, celery leaves, mushrooms, and water to cover in a medium saucepan. Bring to a boil. Reduce heat; cover and simmer 20 minutes or until vegetables are tender. Drain well, and set aside.
Melt butter in a heavy saucepan over low heat; add flour, stirring until smooth. Cook 1 minute, stirring constantly. Gradually add half-and-half; cook over medium heat, stirring constantly, until thickened and bubbly. Stir in salt, pepper, and paprika. Stir celery mixture into sauce.