How to keep all the applications I've opened showing on my external monitor when I close laptop I have a Macbook Pro running Mojave 10.14.4 with an external montior, I have set up mirrored display and made the external monitor my primary display. 

When I close the lid the monitor goes black, as expected, when I press key to get it to light up again all the applications I opened while the lid was opened are not shown on the external monitor. If I open up the lid the monitors go black and I press a key I can see all applications on both monitors. As I hope to just come in, plug in my Macbook  and see what I'm working on in my primary monitor it would be nice if I did not have to open up my secondary monitor to do so.
If you have the displays set to mirrored, they should be identical.

When you disconnect the external monitor, the windows must go somewhere, which is to the built-in display. If it’s mirrored, there can’t be a primary display.