We stumbled on this one  a couple years ago.  We accidentally left our freshly separated cream outside (in an airtight container).  Didn’t find it until the next day when we went back to milk.  We decided to put it in the fridge & check it out later.  Then we forgot about it!  Fast forward a couple weeks – remembered, & checked it.  A beautiful, thick, sour cream!  From that point forward, that’s how we make our sour cream:

Let fresh cream sit at room temp in airtight container for a day.

Place in fridge where it won’t get moved around – allow it to just sit in there for several weeks.

Scrape off thin, top layer & enjoy! 🙂

(We use it from the top first.  It seems the deeper you go, the thinner it is, which is why we don’t mix it all up, either.)