This is a super-simple recipe for an easy appetizer or weeknight dinner. The addition of ginger to the black beans gives it an unexpected zing -- everyone will love it, but won't be able to figure out why. It gets better the longer it sits in the fridge, so give it at least an hour for the flavors to meld together before serving.
It's also good served on its own as a bean-dip appetizer with tortilla chips.
*If you spread the sour cream on the tostada first, it acts like a "paste" to keep all the black beans from falling off. Refried beans and guacamole also do the trick. These things are messy, but oh-so good.