Thanks for reminding me of this Bad/Morally Grey Clark Kent story. Where Clark feeds Bruce something that puts him in a deadly coma for a week. Bruce survives and has nightmares for a bit because Clark fed him mermaid flesh. Eating mermaid flesh makes you immortal if you survive the process. I've been searching for that story for two years!
I can see Diana in collaboration with Clark getting something from Olympus or snatching some magical food from different pantheons to secretly give Bruce to make him some form of immortal.
If you find it, let me know! I’d love to read it. Morally grey Clark is one of my favorite tropes to read.