Put the tomatillos, tomatoes, onion halves, garlic and jalapeño in a medium saucepan. Add enough water to cover all the vegetables by at least 1 inch. Bring to a boil and then simmer until vegetables are tender, but not falling apart, about 10 minutes.
Take the vegetables out of the cooking water and place in a blender jar. Add the piquín chiles and process until very smooth. Season with salt to taste.
Serve at room temperature with your favorite dish.