How should this hole be sealed?

I had an electrician upgrade my main panel and they left this hole were a brick was. I was wondering how I should go about covering/filling the hole? I was thinking insulation and a metal plate to cover it, but wasn't sure if that would be the right way.

I had an electrician upgrade my main panel and they left this hole were a brick was. I was wondering how I should go about covering/filling the hole? I was thinking insulation and a metal plate to cover it, but wasn't sure if that would be the right way.