Add the guajillos, dried pasilla pepper, and the onion to a medium sized saucepan and add enough water to cover everything by a couple inches.

Bring the water to a boil and cook for about 6 minutes, until the peppers have softened. Transfer the guajillos, pasilla and onion to a blender with 3.5 cups of the cooking liquid and add in the garlic cloves, vinegar, chicken bouillon, salt, pepper and thyme.

Blend until smooth.

Pour the mixture into the bottom of a slow cooker or dutch oven, and add in the bay leaves. Place the pork on top of that and cook in the slow cooker on high for 4-6 hours or if using the dutch oven place the pork in the mixture, cover and cook in an oven for 2-2 1/2 hours at 300*F.

To assemble, toast your rolls then add some of the pork to the bottom bun and top with pickled red onions and slices of avocado. (Do not assemble these beforehand or they'll get soggy, you want to retain some of the crunch!)