Probably the most useful technique I've mastered in culinary school was cooking a really gosh-darn-good Bechamel sauce, one of the mother sauces. In my line of work, it's very much a foundational element in many of the recipes we make at Beecher's and Pasta & Co., two brands within the Sugar Mountain family.
The Rouxbe Cooking School posted a really nice video about it, and in the spirit of welcoming the Fall, I offer you this course as a teaser to get back into learning some new cooking techniques.
(if the link doesn't allow direct access, I highly recommend signing up for a preview!)
If you're not already a member of Rouxbe, I encourage you to check out this little preview of classes they provide online. Here's one complimentary video:
Free Cooking School Lesson on Pasta Essentials