What topic sentence can I write to start a research paper on Flannery O’Connor?

I returned this question to you and am glad you clarified it. Since you are writing on Flannery O'Connor rather than a specific work she wrote, one way to begin would be to investigate what motivated her to write the kind of stories she did.
Two possible choices of topic sentences to get you started could be as follows, but keep in mind you will need to adjust your topic sentence to any specific requirements of the assignment and make it your own:
One possible topic sentence: Flannery O'Connor's Roman Catholicism and suffering as a person who had lupus motivated her to show readers that God loves and pours his grace on all people regardless of how terrible they seem to the rest of society.
Alternatively, you might say something like: At first glance, readers might be repelled by Flannery O'Connor's wide array of fictional misfits, but learning about her life can help readers understand how deeply her Catholic faith influenced her stories.
O'Connor's writing is often referred to as grotesque because of how unusual or marginal many of her characters are. Why did she write about racists, misfits, sociopaths, killers, disabled people, ignorant people, and ugly people? How does this connect with her devout Roman Catholicism and suffering from lupus, which killed her at a relatively young age?
Basically, you will be writing about O'Connor's life because she is considered a great American writer. Therefore, you will want to show how her life influenced her work. If you are not interested in her religious faith, you might instead examine how being a Southerner or a woman influenced her writing.