It's a bit of a broad question for SO I think. But here are some thoughts. In my opinion, pure unit testing should mock APIs programmatically and not depend on external applications like Mockoon.

What you want to do seems to be more like integration testing, where your code is calling an external API, except that instead of maintaining an instance of the original API specifically for test/QA purposes, you maintain a mock (with Mockoon).

Then, the general idea is more or less the following:

• you prepare the mock API with Mockoon locally with the UI.
• you save Mockoon's data file in the repository.
• During the GitHub Actions workflow, you install Mockoon CLI and start it targeting the data file (relevant tutorial).
• You run your application tests and modify the environment variable containing the API URL to point to were the CLI is running (probably something like http://localhost:3000).

[Update 2023-11-20] Since version 3.1.0 a recording mode has been added.

There is no "automatic" recording option in Mockoon. But you can enable the proxy and from the "Logs" tab add the endpoints manually.

1. Enable the proxy:

2. Manually mock an request/response: