Begin by making the Spicy Pulled Pork: Preheat the oven to 300°.
At the bottom of a large Dutch oven, place the sliced onions. Then put in the pork butt, fat side up. Mix all the spices and spread them all over the pork. Add chicken broth and garlic to the bottom of the pot.
Bake, tightly covered, for 5 hours.
Shred the pork with two forks, and stir in the sugar. You will have lots of leftover pulled pork for another use. This pork freezes great and see this blog for lots of other recipes using this pulled pork.
While the pork braises, make the pickled onions: Simply stir together all the ingredients and chill until serving.
Also, bake the tostada shells: Preheat the oven to 400°.
Rub tortillas on both sides with olive oil and bake for 10 minutes on a baking sheet.
Top the tortillas with pulled pork, queso fresco, cilantro, avocado, and pickled onions. Serve with the lime wedges.