Stem and seed the guajillo chiles, then add them to a pot of water with the onion and garlic. Bring it to a low boil, then turn the heat down to low for 10-15 minutes, or until the chiles are soft.
Drain the chiles, onions, and garlic, then transfer to a blender. Add in ½ cup of cooking water or vegetable broth and a generous pinch of salt. Blend on high until a paste is formed, adding more liquid as needed. Strain into a bowl and set aside.
In the meantime, scrub and cube the potatoes into ½-inch pieces. Steam or boil them for 10-12 minutes, or until tender when pierced with a fork.
Heat about ½ tablespoon olive oil in a large skillet over medium. Add in the potatoes and cook for 2-3 minutes, then mix in the vegan chorizo.
Season with salt and cook for an additional 3-4 minutes, or until the chorizo is hot. If your potato chunks are bigger, roughly mash them, then transfer everything out of the skillet.
Heat ½ – 1 tablespoon olive oil in the same skillet over medium. Cut your pambazo bread in half, leaving the top partially attached. Brush one side with guajillo paste, then add it to the skillet paste side down. Press down on the bread with your spatula to make sure the whole surface is cooked.
While the first side is cooking, brush the bottom surface with more paste. Fry each side of the bread until crispy. Continue this process with the rest of the buns, adding more oil to the pan as necessary.
Add a scoop of chorizo and potatoes to the pambazo bread, then top with lettuce, salsa verde, Mexican crema, and queso cotija. Serve immediately while warm. Happy eating!