Assign a mentor and give the onboardee free reign over them. Make sure that the mentor is okay with it and make it clear to them that this can take as much out of their time as needed and it won't reflect badly on their perceived performance. If need be, allow them to be unavailable for X specific hours a day to allow for uninterrupted work on important tasks but otherwise remain totally there for the onboardee.

Accept that your employees won't be productive from day one, and they don't have to. If your employees can be productive from day one on tasks, you're probably postponing the real onboarding and making yourself feel good.