Take any little stems off the tops of the peppers. Cover them with boiling water. Let them soak for a half hour.
Pour the chipotles and the soaking water into the food processor. Add the tomato paste and sweetener and process until a paste forms. Add to a small pot.
Slice the onions into half moons. Add them along with the garlic, vinegar, salt, and peppercorns to the pot.
Bring to a boil and then reduce to a simmer. Cook, uncovered, for about an hour and 30 minutes to an hour and 45 minutes. It will reduce down significantly and become very thick. Check at the 1:30 mark and keep cooking if you want it thicker.
Let it cool completely. Run through the food processor to get it as smooth as you can.
Store in a jar in the refrigerator or in ice cube trays in the freezer. Enjoy!