In a large bowl, dissolve the salt and sugar in 8 cups of cold water. Add the black peppercorns and squeeze in 2 of the halved lemons. Submerge the pork in the brine for 6 hours.
Preheat the oven to 450 degrees F. Remove the pork from the brine and dry off. Heat a large oven-safe skillet (preferably cast iron) over medium-high heat. Add the oil and sear each side of the loin until golden, about 8 minutes total -- keep turning until it's golden all over. Add the sliced onions to the pan alongside the pork, and stir around to coat the onions in fat. Transfer the skillet to the oven and roast the pork until it reaches an internal temperature of 140 to 145 degrees F, about 30 minutes. For even cooking, be sure to stir the onions once or twice during roasting.
For the sandwich build: Butter the pretzel bun halves and griddle them until golden brown. With a sharp knife, slice the loin into thin shavings. Spread some mustard on the bottom half of each bun, then top with some sliced pork. Sprinkle the pork with a little salt and squeeze the remaining lemon halves over the top. Top with the roasted onions and some Pickled Fresno Peppers. Eat while standing.
Pickled Fresno Peppers:
Bring the vinegar, 1/4 cup water, the sugar, salt, bay leaf and garlic to a boil in a small saucepan and simmer for 5 minutes. Pour the hot liquid over the peppers in a bowl or container. Cool to room temperature and refrigerate.