Roast tomato, onion slices and jalapeno on an open fire until charred and slightly soft. I do this on a rack placed over my burner on my gas stove. The tomato will split and start to juice. Oh well. Just let it char. This is what my Mexican friend did. It does work. (a little clean up later) I also thread the onion slices on a metal skewer. He did not, but I do. They start to fall apart. Watch this every second and turn vegetables as needed. You could do this on a gas grill or maybe in the broiler? Never tried that. Anyway, this step is important.
After roasting vegatables, take stem off of pepper. Add whole pepper, whole tomatoes, and onion into a blender while still hot. Put in cilantro and squeeze half the lime into the blender as well.
Pulse until desired consistency. Taste and add seasonings and add more lime if needed. It gets better as it sits.
Simple ingredients. Simply delicious. Serve as a condiment over grilled meat or tacos, burritos, chips, whatever.
Do not leave out the roasting. It makes the sauce. Find a way to do it.
If this is not hot enough, or you think it will not be, prepare another jalapeno when roasting vegetables and use it in step 2. I never saw my friend use more than one.