1) Universal Workload [Enterprise only]
If you have the chance to have an Enterprise account, you will find happiness with the new feature Universal Workload.
2) Using Portfolio Workload
As a (very) manual workaround, you could do an advanced search, and search for all tasks where your employee is assigned to.
And look at all the projects of the column Projects
And add all those projects manually to a dedicated Portfolio (to ultimately look at the Portfolio Workload).
(Tips) You can sort your advanced search by project (it will create one section per project)
Proactively, you can make sure all projects are always added to a certain portfolio via the template (at the condition, everyone is always using certain templates to create projects).
3) Using Reports
Less fancy than the Portfolio Workload view, Reports are still a great way to look at workload
Hope that could help 