Hi there.
This feature is not currently supported in our EF Core provider - it’s part of the “Includes/joins” limitation of the provider listed in the README.
If you configure the EFCore.Proxy package you might have some success but bear in mind that they are loaded on-demand as you access them with another query rather than a join/$lookup so you may suffer performance issues.
You can keep an eye on https://jira.mongodb.org/browse/EF-117 which is the include/join/$lookup work - it’s currently in the investigation phase and being actively worked on.