In a stock pot, add guajillo chiles, chile de árbol, onion, garlic, and water.
Cover with a lid and bring to a boil.
Turn heat off and wait for 5 minutes.
In the meantime, roast your tomatoes over a griddle or place under the broiler for a few minutes.
Char the tomato skin on all sides.
Add the tomatoes, guajillo chiles, chile de árbol, onion, garlic, and cooking water to a blender.
Blend until smooth.
Heat oil in a stock pot.
Place a strainer over the pot.
Carefully strain the liquid from the blender into the pot.
Using the back of your cooking spoon, push all the liquid through the strainer.
Cook sauce for 5 minutes. Stir frequently.
Warm up the leftover pork.
To assemble your sandwich, using a deep plate or bowl, cut the bread in half.
Add the pork to one half and top with the other half.
Ladle a spoonful of sauce over the sandwich.
Add more sauce around the sandwich.
Top with pickled red onions.
Enjoy.