Finely slice the 3 red onions and cut the 250 g aged guanciale into small strips.

Bring 4 l water to a boil in a large pot and salt the water lightly.

Cook the 350 g spaghetti in the boiling water until very al dente, about 2 minutes less than the package directions.

Cook the guanciale strips in a cold large skillet over medium heat until the fat renders and the meat becomes crisp.

Add the sliced red onions to the rendered guanciale fat and cook until deeply browned and lightly charred.

Pour 15 ml vinegar into the skillet and scrape up the browned bits from the pan.

Reserve 240 ml pasta water, then drain the al dente spaghetti.

Add the drained spaghetti to the skillet and toss it with the guanciale, onions, and about 120 ml reserved pasta water.

Remove the skillet from the heat and stir in the 250 g finely grated Pecorino Romano cheese until creamy.

Season with black pepper and a little salt if needed, then serve immediately.