1. Cook the spaghetti according to the package instructions. Dissolve the bouillon in a bowl of water. Add soup, celery salt, and pepper. Drain spaghetti; add soup to mixture. Stir in chicken, onion, pimentos and 1/2 cup of cheese.
2. Transfer to a greased 8-inch square baking dish. Top with the remaining cheese.
3. Bake, uncovered, at 350 degrees F for 35-40 minutes or until heated through.