Here are two thesis (topic) sentences. One is about the pro of frozen food and the other is about the cons. :)
Pro:
1. Frozen food has become a staple in almost all homes in the U.S. because of its convenience and shelf life. It could be considered one of mankind's greatest inventions.
Against (con):
2. Frozen food, although convenient, is one of the many things contributing to health issues in the U.S. It's high sodium and calorie content is detrimental to our health.
I hope this helps! If you have further question, leave them below! :)