Castaway Getaway by PeaceHeather
This was a slowburn frostiron fic, and for whatever reason Odin and Frigga had exiled Loki to an alien planet -- lots of pristine wildlife, zero people -- where he had to hunt food and gather shelter to survive. It was maybe Loki's humility test? At some point, Tony is exiled with Loki. Both of them started looking at this exile as a massive extended vacation -- no one to bother them, enjoyable company, heckin' fun engineering projects.
There were some serious Minecraft vibes to the story, and a lot of detail about using hand-made tools to build a forge, and then using the forge to build better/more specialized tools, and then using specialized tools to build a better house, etc.
The story ended with Loki released from the exile and offering Tony an apple, in a scene along these lines:
Tony: This isn't one of those golden apples conveying immortality, is it?
Loki: Heck no, look at how normal it appears.
Loki: By the way, I'm going to need that apple core back.
I think I read this one on Ao3, but my googling is not turning it up. Does it sound familiar to anyone? THANKS!