how do i do a clean El Capitan install after installing Sierra

l installed Sierra and found that Safari crashes immediately after start, which I reported in another post. Then I reverted to El Capitan using a Time Machine backup and somehow my Apple ID login has gotten corrupted. i.e. it works fine from all other devices except the iMac running El Capitan. I opened a case with Apple Support about that and after 4 days it still is not fixed, so I want to do a clean El Capitan install, create my login from scratch, then restore apps, etc. from Time Machine. The problem is that in recovery mode, when I select reinstall Mac OS, the only option it gives me is to install Sierra. How do I override that? I am currently running El Capitan, so I am surprised that the Mac OS version that the recovery mode knows about is Sierra. Something did not get reset when I reverted to El Capitan from Sierra.