Begin by making the adobo sauce: Remove the seeds and stems from the peppers and then toast them in a cast iron skillet on high heat for a few minutes, until you can smell their aroma.
In a three quart sauce pan boil the peppers along with two cups of water, for five minutes. Drain the peppers, and then put them in the blender along with two cups of fresh water, the vinegar, oregano, cumin, cinnamon, and cloves. Blend until the mixture is smooth.
Preheat the oven to 300 degrees.
For the Birra beef topping: Dry off the short ribs and sprinkle them with the salt, pepper, and garlic powder. Heat the olive oil in a Dutch oven on medium heat. Sear the ribs, a few at a time, on all sides. When they're all browned add them all back to the pot along with half the adobo sauce (you can freeze the rest for another use), the chicken stock, the bay leaves and the thyme. Cover the pot and bake them for three hours, or until they're very tender.
While the beef braises, make the Pickled Onions: Simply stir all the ingredients together in a bowl.
Also, right before serving, bake the tostada shells: Preheat the oven to 400 degrees. Brush the tortillas on both sides with the oil and bake on a sheet pan for ten minutes.
Remove the meat from pot and shred it. Pour the liquid in the pot into a measuring cup. The fat will rise to top. Remove and discard the fat. Put the beef in a bowl and stir in some of the liquid so that the beef is moist but not real drippy.
To assemble the tostadas: top each tortilla with some of the beef, cilantro, queso fresco, and pickled onions. Serve with lime wedges.