Place the thinly sliced red onion in a bowl. In a separate jug, stir together the vinegar, hot water, sugar, kosher salt, and lime juice until the sugar and salt dissolve. Pour over the onion and let sit for 20–30 minutes, tossing once or twice. If using chili and cilantro, add them after the onion has softened slightly. The onion should become bright pink, glossy, lightly softened, and sharp but balanced. It should still have bite, not turn limp like fully cooked onion. Using hot water to dissolve the sugar and salt into the vinegar brine, rather than cold, is what lets both dissolve fully and quickly before the mixture is poured over the onion — undissolved granules left in a cold brine would sit unevenly rather than seasoning the pickle consistently.

Cut the pork belly into large thick pieces that will be easy to slice after cooking. Place the pork in a pot and cover with water. Add 1 tsp fine sea salt for simmering. Bring to a simmer and cook gently for 45–60 minutes, until the pork is tender but not falling apart. Remove the pork from the water and dry it extremely well. Let it steam-dry for at least 10–15 minutes. The pork should be tender enough to slice but still structured. If it falls apart before frying, the sandwich loses those thick clean chicharrón slices. Drying is critical. Wet pork belly will spit violently in hot oil and will not crisp cleanly. Stopping the simmer while the pork is tender but still holding together, rather than pushing it toward fully falling-apart tenderness, is a deliberate choice specific to this sandwich — pan con chicharrón depends on thick, clean, sliceable pieces rather than shredded meat.

Season the dried pork belly with the 2 tsp fine sea salt, garlic powder, onion powder, and black pepper. Let it sit while you prepare the sweet potato. The seasoning should coat the pork evenly, especially the meat sides. The skin/fat side will crisp from frying, but the meat needs seasoning now. Concentrating the seasoning on the meat sides, rather than trying to season the skin evenly too, reflects a real difference in how each surface will ultimately taste — the skin's flavour comes primarily from the frying process itself, while the meat has no other opportunity to pick up seasoning before it's sliced and served.

Peel the sweet potatoes and slice them into thick rounds, about 1–1.5cm thick. Simmer the slices in lightly salted water for 6–8 minutes, just until barely tender. Drain and let them dry well. Fry or shallow-fry in neutral oil or lard until golden on the outside and soft inside. Season lightly with salt. The sweet potato should be golden and creamy, not mushy. It needs to hold its shape inside the sandwich. Simmering the sweet potato only until barely tender, well short of fully cooked, gives it structural insurance for the frying stage that follows — slices already fully soft from simmering would risk falling apart or turning mushy once they're fried further.

Heat neutral oil or lard in a heavy pot or deep pan to about 175–180°C. Fry the pork belly pieces in batches until deeply golden, blistered, and crisp on the outside, about 5–8 minutes depending on size. Transfer to a rack and season lightly with extra salt while hot. Rest for 5 minutes, then slice into thick sandwich pieces. The outside should be crisp and browned, while the inside stays juicy. If the oil is too cool, the pork turns greasy. If too hot, the outside burns before the fat renders properly. This specific temperature window is a genuine balance point for pork that's already been fully cooked through by the earlier simmer — the fry's only remaining job is rendering and crisping the exterior, which needs enough heat to happen efficiently without scorching the surface before that rendering completes.

Split the rolls open. Warm them briefly in the oven or toast the cut sides lightly so they stay crusty but not dry. The roll needs enough crust and structure to hold pork fat, sweet potato, and onion liquid. Very soft bread collapses too fast. A genuinely crusty roll, rather than a soft sandwich bun, is what this recipe specifically calls for — the sandwich's substantial fillings, several of which carry real moisture and fat, need bread with enough inherent structure to withstand that load.

Place fried sweet potato slices on the bottom half of each roll. Add thick slices of crispy pork belly chicharrón. Top generously with the pickled red onion / salsa criolla, letting some onion spill from the bread. Close the sandwich lightly and serve immediately. The final sandwich should feel abundant and slightly messy. You want visible pork, orange sweet potato, and bright pink onion all coming through the roll. Building the sandwich in this specific order — sweet potato first, then pork, then onion on top — places the driest, most stable component at the base and the wettest, most acidic component last, minimizing how long the bread beneath is exposed to the pickled onion's liquid before serving.