Piedmont — the Asti and Cuneo provinces are the centres of Corno di Bue pepper cultivation. The peperonata preparation is documented in Piedmontese summer cooking from at least the 19th century.
Piedmont produces some of the finest sweet peppers in Italy — the Corno di Bue (bull's horn) peppers of the Asti area, thick-walled, sweet, and bright red, roasted or braised into the definitive peperonata: a long, slow stew of peppers, onion, tomato, and olive oil that reduces over an hour into a dense, sweet, slightly jammy preparation that is simultaneously a condiment, a side dish, and a sauce for meat. The Piedmontese peperonata is notably slower and denser than southern versions — the peppers are cooked until they nearly dissolve. It is one of the fundamental preparations of the Piedmontese summer table.
Piedmontese peperonata at its correct long-cooked consistency is almost a jam — the sweet peppers, reduced to a dense, orange-red stew, have a concentrated sweetness modulated by the slight acid of the vinegar. The onion has dissolved; the tomato is present but not dominant; the olive oil binds everything. It tastes of late summer preserved in a pan.
Use thick-walled sweet peppers (Corno di Bue or equivalent). Roast over a gas flame or under a grill until completely charred on the outside. Peel, remove seeds, tear into strips. In olive oil, soften sliced white onion over low heat for 15 minutes. Add the pepper strips, crushed peeled tomatoes, salt, and a small amount of white wine vinegar (the acid note is the distinguishing Piedmontese touch). Cook over very low heat, uncovered, for 45-60 minutes, stirring occasionally, until the peppers are completely tender and the sauce has reduced to a dense, slightly jammy consistency. The peppers should be almost falling apart.
The white wine vinegar is the Piedmontese signature — just a tablespoon or two, added after the initial soffritto, provides an acid counterpoint to the sweet peppers. The finished peperonata improves after a day or two as the flavours integrate. It is an excellent companion to bollito misto (the Piedmontese boiled meats) or with grilled chicken.
Not roasting the peppers before braising — the char and subsequent peeling removes the tough skin and adds depth; raw peppers produce a completely different preparation. Cooking at too-high heat — the peppers burn or the liquid evaporates before they fully cook down. Insufficient time — 45 minutes minimum is needed for the correct jammy consistency.
Marcella Hazan, Essentials of Classic Italian Cooking; Slow Food Editore, Piemonte in Cucina