"My introduction to black bean soup was served with a slice of lemon floating on its rich brown-black surface. Following my host's lead, I sank the lemon with my spoon and smashed it against the bottom of my bowl, releasing a snappy hit of lemon juice. My version of this smooth and elegant soup uses a combination of lime (instead of lemon), cumin, and chipotle chile for a piquant Mexican savor. It's a delight either hot or cold...."