Place whole squash in a medium saucepan with boiling water to cover.
Cook 8 minutes or until tender but firm.
Drain squash well, and let cool to touch.
Cut squash in half lengthwise; remove and reserve pulp, leaving a firm shell.
Set aside.
Chop pulp, and set aside.
Coat a small skillet with cooking spray; place over medium-high heat until hot.
Add onion and parsley to skillet, and saute until onion is tender.
Stir in squash pulp, breadcrumbs, egg, pimiento, salt, and pepper.
Spoon mixture into squash shells, sprinkle each with 1/2 teaspoon Parmesan cheese.
Place in a baking dish coated with cooking spray.
Bake, uncovered, at 350° for 20 minutes or until thoroughly heated.