Preheat oven to 350 F. Slash squash in several places with a small knife. Place squash on an ungreased baking dish. Bake 1 hour or until fork inserts easily. Cool 15 minutes
Meanwhile, in a heavy saucepan, melt butter over moderate heat, add the onion, celery and pepper, cook until onion is soft, about 5 minutes. Slowly add flour to vegetables, stir, add chicken broth, milk, and bay leaf. Cook 20 minutes, stirring occasionally. Discard bay leaf, stir in sour cream, parsley and lemon juice. Remove from heat.
Halve spaghetti squash, remove seeds. Scrape the flesh into ungreased 1 ½ quart casserole, smoothing into an even layer. Cover with chicken, then the sauce, top with bread crumbs. Bake uncovered for 30 minutes or until bubbly. Serve with green beans and cherry tomatoes.