Prep the vegetables, and add to a medium saucepan.
Add the other ingredients and cook over medium heat, stirring occasionally, until the tomatoes and onions are cooked, approximately 10-15 minutes.
Blend half the sauce in a blender or food processor, or carefully use a stick mixer to blend half the sauce.
Transfer to a bowl and refrigerate. Serve cold.
Notes
This will keep for ten days in the refrigerator. Adjust the amount of jalapeño to suit your heat level.If you can, use a pure chile powder such as guajillo or pasilla, as opposed to regular chili powder, which contains other ingredients beside ground dried chiles.