Sleep mode with external monitor

Using a Macbook Pro with external monitor - both screens used.

Used to be on El Capitan (yeah, so bite me - that was the OS with the last acceptable UI for iTunes) and I added an external monitor a couple of years ago at which point going to menu and selecting Sleep worked (slept both screens) but closing lid just slept Macbook screen, not Macbook itself, so external screen stayed on, with all windows gathered to it. I believe this is called clamshell mode. I did eventually find out how to get both screens to sleep when I close the lid. Can't recall how.

Upgraded to Sierra (so bite me again) and now the same thing is happening but there is no obvious setting in System Preferences to fix this again. And I cannot recall what hoops I had to jump through last time.

I want to NOT work in clamshell mode. Ever. It appears clamshell mode is default, so closing lid when an external screen is attached forces it into clamshell mode. Why is there not a 'clamshell mode' yes/no setting in System Preferences? If there is, where is it and what is it called?

(Anyone replying with comments about how I need to upgrade or about there being settings in later versions will be ignored. I know this was possible with El Capitan - I did it - and I see no reason for it not to be replicable on Sierra.)