Turn heat down to medium and add the onion and garlic. Cook for 5 minutes then season with cumin, oregano, salt and pepper. Deglaze the pot with the beer, scraping up the bits stuck to the bottom of the pot. Add the chicken stock and orange juice, and stir. Add the pork back to the pot -- the liquid should come 2/3 the way up the meat. Bring to a simmer, then cover and place in the oven for 2 1/2-3 hours, until the meat is tender and falls apart when you touch it, turning the meat halfway through cooking time.
Remove the pork from the pot, place on a plate and tent with foil to keep warm. When it is cool enough to handle, pull the meat apart with 2 forks.
Simmer the braising liquid over medium heat until reduced by half. Add the juice of two limes. Set the shredded pork into the liquid.
While the pork is cooking, prepare the pickled red onions: In a small saucepot, bring the vinegar, sugar, a teaspoon of salt, chili or jalapeño pepper and bay leaves to a simmer. Place the sliced red onions in a small sealable container. Carefully pour the hot liquid over the onions, cover and give it a shake. You may need to lift the lid to release some of the pressure. Every now and then, give it a good shake. Place in fridge to cool.
To assemble sammies, remove pork with tongs from the liquid and place on bun bottoms. Top with a few pickled jalapeños, some pickled red onions, some crumbles of queso fresco and top with the bun tops.