Preheat the oven to 350 degrees F (175 degrees C). Grease a medium baking dish.
Melt 4 tablespoons butter in a medium saucepan over medium heat. Add celery and cook until tender, about 5 minutes. Move celery to the side and mix in flour and salt. Pour in milk and cook, stirring constantly, until thickened.
Stir in mushrooms, bell peppers, and pimento peppers, then stir in cheese until melted.
In a small bowl, blend bread crumbs and 2 tablespoons melted butter. Transfer celery mixture to the prepared baking dish and sprinkle bread crumb mixture over top.
Bake in the preheated oven until lightly browned, about 20 minutes.