Keep the barbacoa at a low temperature, with some of its own broth, so it stays warm while you prepare the toppings and sides.
Dice the onion and tomatoes into small pieces, chop the cilantro finely, and cut each lime into four equal wedges.
In a medium pot, bring water to a boil over medium-high heat. Add onion, garlic, and pepper and simmer for 10 minutes or until the vegetables are soft. Add the tomatillos and simmer for 3 to 5 more minutes, just long enough so that they change color from bright green to pale green. Use a blender or food processor to puree the vegetables and cilantro, add salt and pepper to taste. Adjust the consistency with some of the boiled water. I added 1/2 cup of water.
In a medium pot, bring water to a boil over medium-high heat. Add roma tomatoes, onion, garlic, and chilies. Reduce heat to medium and simmer for 10 minutes or until the vegetables are soft. Use a blender or food processor to puree the vegetables. Add the dry oregano and season with salt and pepper to taste. Adjust the consistency with some of the boiled water. I added 1/2 cup of water.
Wrap warm tortillas in aluminum foil and a kitchen towel as soon as they come out of the skillet to keep them soft and pliable.
Assemble barbacoa tacos and top with your favorite toppings.
Enjoy!