How do I make the beef in my beef stroganoff more tender?
Supermarket 'stew beef' is notoriously unreliable.  Its often just scraps of beef that the butcher or market can't sell otherwise.  My stews were hit and miss for years while I tried to tweak cooking time and such. But then one day I watched a movie on rouxbe.com (paywall, sorry) and they talked about not using 'stew beef' - its often too lean and doesn't have the 'stuff' to produce tender, but juicy bits.  They strongly recommended cutting up your own from a chuck....and they were so, so, so right.

In short, cutting up your own chuck is by far the better route - if you're looking for 'stew beef'.