Preheat oven to 350°.
Wash squash; cut in half lengthwise. Remove and discard seeds. Place squash, cut sides down, in a 13- x 9-inch baking dish coated with cooking spray. Add water to depth of 1/2 inch. Bake at 350° for 45 minutes or until tender when pierced with a fork; let cool slightly. Using a fork, scrape spaghetti-like strands onto a platter; set aside, and keep warm.
Coat a saucepan with cooking spray; add oil. Place over medium-high heat until hot. Add leek; cook, stirring constantly, 3 minutes or until tender. Add beans and tomatoes; cook over medium heat 5 minutes. Stir in olives and next 3 ingredients; cook until thoroughly heated. Spoon bean mixture over squash. Garnish with celery leaves, if desired.