Replaced logic board on Macbook Pro, won't boot from Mac OS, only from Win on BootCamp

Hello good people,

The problem I got myself into is as follows:

I had to have the logic board replaced on my 2012 MacBook Pro 13'' after liquid spillage. I have Boot Camp on it as well, with Win 7.

Now it won't boot from Mac OS, it only starts in Windows. Boot camp / Win works perfectly, no issues there; it will also start in recovery Mode using cmd+R, but the options are limited and I could not find anything there to get it running in Mac OS again.

I am not too fussed about recovering anything at this moment, I have most docs saved on an external drive, I just want to format and start with a fresh install if I cannot get it back on track as before.

Could someone help with some tips / instructions on what I could do here?

Thank you!

Dan