I currently have a global function in my functions.php file

ein_error_log($message)
{
      //push out $message to file... 
}

But I want to start using it in my MU-Plugin directory and it doesn't know it exists. So I assume it's because MU-Plugins folder is read before the theme folder in WordPress's hierarchy, which makes sense.

But if the hierarchy is the case, how do I distinguish which MU-Plugin runs first to make sure all other MU-Plugin files can run dependent on said file?