Extract Boot to Recovery HD by holding down Command+R
Connect external disk (minimum 10GB)
Select Reinstall Mac OS X and target external disk
Installer completes and restarts
Disconnect external disk and hold Option (Apple Boot Menu)
Boot from regular internal boot drive Reconnect external drive
Navigate to Mac OS X Install Data
Copy InstallESD.dmg
Download from App Store
Show Package Contents
Navigate to SharedSupport
Paste in InstallESD.dmg