In a large skillet, saute celery in 4 tablespoons butter until tender, about 5 minutes. Remove celery with a slotted spoon and set aside.
Stir flour and salt into skillet until smooth. Gradually add milk; bring to a boil. Cook and stir for 2 minutes. Add mushrooms, green pepper, pimientos and cheese. Stir until cheese is melted. Return celery to pan; stir to coat.
Pour into a greased 1-qt. baking dish. Melt remaining butter; toss with bread crumbs. Sprinkle over the celery mixture. Bake, uncovered, at 350° for 20-25 minutes or until bubbly.