Cook pasta in a large pot of boiling water until al dente. Drain.
Meanwhile prepare the sauce. Heat oil in a medium saucepan over medium heat. Add onions and cook until onion is transparent, 2 to 3 minutes. Add garlic and cook for 30 seconds. Stir in thawed spinach. Mix in sour cream, cream of celery soup and Parmesan cheese. Reduce heat to low, and heat through.