To prepare the pickled onions, slice the onion into thin half-rounds and transfer to a nonreactive bowl. Add the vinegar, lemon zest, lemon juice and sugar and stir to combine. Cover with plastic wrap and refrigerate overnight. The next day, stir in the salt. The onions can be stored in the refrigerator for up to 2 weeks.

To prepare the pork, in a large sauce pot over high heat, combine the stock, onion, garlic and red pepper flakes and bring to a boil. Add the pork, then reduce the heat to low and simmer until the pork is tender enough to pierce with a fork and begins to fall apart, 1 1/2 to 2 hours. 

Remove from the heat and let the pork cool in the juices for 3 hours. Transfer the pork to a cutting board. Pull off any excess fat and discard, then using a fork, shred the meat. Set aside.

To prepare the tostadas, in a large sauce pot, heat the oil to 375°F on a deep-frying thermometer. 

Drop the tortilla triangles into the hot oil and fry until lightly browned, about 1 minute. Transfer to paper towels to drain. Sprinkle with coarse salt and set aside.

To prepare the Sriracha aioli, in a bowl, stir together the mayonnaise and Sriracha hot sauce. If desired, transfer to a squeeze bottle.

To assemble the tostada bites, add 1/2 tablespoon Sriracha aioli to each tostada triangle and place 1 tablespoon warm pulled pork on the aioli. Top with some pickled onions and sprinkle each with 1 teaspoon Cotija cheese, some cilantro and a squeeze of lime juice.