Cut squash in half; discard seeds. Place squash, cut side down, in an ungreased 15x10x1-in. baking pan. Fill pan with hot water to a depth of 1/2 in. Bake, uncovered, at 350° for 40 minutes.
Meanwhile, in a skillet, saute celery and onions in oil until tender. Add the spinach and 1/4 teaspoon salt; cook and stir until spinach is wilted.
In a bowl, combine the bread crumbs, pecans and remaining salt. Drain water from baking pan. Turn squash cut side up. Stuff with spinach mixture; sprinkle crumb mixture over top. Dot with butter. Bake 15 minutes longer or until the squash is tender.