Grill the tomatoes and jalapenos until the skin is blackened.
Let cool.
Cut out the core of the tomatoes and throw everything else into a food processor (discard the core).
Use the blunt side of a knife and scrape the skin off the jalapenos. Cut them lengthwise and open them up, remove the core and seeds, and add the flesh to the food processor.
Add half bunch of cilantro and pinch or two of kosher salt (table salt can work..)
Add a couple cloves of peeled, crushed garlic.
Run the food processor until at a consistency you like.
Taste. If you like it hotter, add the jalapeno seeds. If it's too hot, add some citrus juice (like a Tbsp. at a time)
Put in a serving or holding container and pop it into the refrigerator for about an hour.
Serve cool.
ETA: dunno if it's authentic at all, but it's darn good. I will usually add half of a yellow onion (grilled) to the recipe.