This is your dream assignment. You can't wait to jump in. You've actually been reading about related topics on your own and have some ideas about where you want to take this. You write out a bit of an outline, figure out what resources you've alread read, where the remaining questions are, and get to work finding the answers to those. While there is maybe more time spend reading for this essay than most others that you've written, you're OK with that because it is a topic you already care about.
In the end, you write a very thorough paper. The biggest challenge is in keeping it focused and not writing too much. You're not sure if it is a great paper because you end up with even more questions that you don't have time to pursue, but it sure was fun to write!
Likely, any reservations that the student has about the quality of the paper are not ones that you as a teacher have. You probably have writing that shows evidence of deep thinking based on more reading than you typically see from essay submissions. This outcome has been achieved completely by chance however. Developing personal connections to a topic are important for students to be able dive deep and develop a true understanding of that topic. Personal knowledge comes from fitting new ideas into what one already knows or has experienced. Often, heavily prescribed essay topics don't take this into account and ask students to create new knowledge independent of what they already know.