Preheat the broiler.
Put the tomatoes, garlic, onion and chile on a baking sheet and broil 5 minutes. Pull the baking sheet out and turn everything over.
Put the baking sheet back under the broiler and broil another 5-7 minutes. The tomatoes should be charred and a bit mushy.
NOTE: If you don't want to broil them, you can char everything in a cast iron or other heavy skillet.
Take the skin off the garlic clove then it in a food processor, along with the onion slice, the tomatoes and any juices, salt, and half of the jalapeno.
Blend until smooth and then taste for heat. Add the other chile if you want it hotter.