Onboard a new developer in a week, not a month
Prepare in advance
An orderly access list activated on day one; a local environment that comes up with one command; and dummy data that lets you work without touching real data.
Every hour wasted on permissions is an hour not spent understanding the system.
The right first task
Small, real, and touching the whole chain: code, test, review, deploy. The goal isn't the output but going through the path once with guidance.
Assign a fixed mentor for two weeks, with a set time on the calendar and not “ask if you need to” — most people won't ask.
Learn from the process
Ask the newcomer to fix the documentation in real time. A first look at a system is an asset that vanishes after two weeks and doesn't return.
Going deeper
Keep a one-page system map: the components, the main flows, and the places where most incidents occur. It's worth more than dozens of pages, and easy to maintain because it changes slowly.