Melt butter in a medium saucepan over medium heat, and cook celery until tender, about 5 minutes. Move celery aside, and mix in flour and salt. Pour in milk, and mix all ingredients, stirring constantly, until thickened.
Stir mushrooms, green bell peppers and pimento peppers into mixture. Mix in cheese, and stir until melted.
In a small bowl, blend bread crumbs and butter. Transfer the celery mixture to a medium baking dish, and sprinkle with the bread crumb mixture. Bake 20 minutes in the preheated oven, or until lightly browned.