Sure, anybody can make a two-hour plein air painting, but taking that painting and turning it into a large, finished studio piece is another matter entirely!  "Just use a bigger brush" some might say.  (Or, in this case, a bigger piece of pastel.)  Going "large" involves refining composition, taking the time to correct errors that were made on-location, and more.  As a teaser, I'm posting my plein air piece plus the finished studio piece below.  You can see the full demo at my web site.
Here's the plein air piece (9x12):

And here's the finished studio piece (19x25):