How do you approach an otherwise healthy patient with an incidental 1 mm GIST involving the serosal surface in terms of staging workup, EGD, and surveillance?

These incidental "micro-GISTs" are not likely to become a clinical problem. Interestingly, you describe a "serosal" lesion which would not get picked up on endoscopy anyway, but not unreasonable to check again in a year with a CT AP and EGD. I don't think we have any evidence to support continued long-term surveillance in this scenario, and reverting back to usual annual PCP visits should be fine. If there are any new symptoms, those would then direct the workup.

We will need more information like location and mitoses per 50 HPF (or 5 mm). Please use the Sloan Kettering nomogram to determine the risk of relapse. Seems like a very low risk of relapse. NCCN recommends 6-month scans for the first 2 years and then once a year to finish 5 years. There is no need for EGD. We usually do CT CAP with contrast for surveillance.