1. Blister the jalapeños in a small, dry skillet (I usually use a cast-iron pan) over medium-high heat, making sure you flip them occasionally for a relatively even blister, five to seven minutes. Cut off and discard the stems. Transfer the chiles to a blender jar.
2. Sprinkle the cumin seeds into the hot pan and allow them to toast until they smell nutty, barely five seconds. Add them to the chiles. Pour in the cream and add the tomato and salt.
3. Puree the medley into a smooth sauce, scraping the inside of the jar if needed to ensure an even blend.