There’s a little corner of sandwich heaven hiding in plain sight in Lisle, Illinois, where Schmaltz Delicatessen has been quietly perfecting the art of the Reuben while flashier eateries come and go.

The matzo ball soup is a particular standout—clear, flavorful broth surrounding matzo balls that are the perfect consistency, neither too dense nor too light, seasoned just right and comforting in a ...