Render the base

1. Set a large skillet over medium heat. Add the guanciale and let it render slowly for 6 to 8 minutes, stirring now and then, until the fat is glossy and the edges are browned; the pieces should look crisp outside but still slightly chewy in the center.
2. Transfer about one-third of the guanciale to a plate for finishing. Add the chili flakes to the skillet and stir for 15 to 30 seconds, just until fragrant.
3. Add the crushed tomatoes and their juices carefully; the pan may sputter. Stir well, scraping up any flavorful bits on the bottom.
4. Bring the sauce to a lively simmer, then lower the heat to medium-low and let it reduce for 8 to 12 minutes, stirring once or twice, until it looks brick red and lightly glossy rather than watery.

Boil the pasta

1. Meanwhile, bring a large pot of water to a boil and salt it generously. Add the spaghetti and cook 1 minute less than the package directions, usually 8 to 10 minutes, stirring early so the strands do not stick.
2. Scoop out 1½ cups / 360 ml pasta water, then drain the spaghetti.

Finish the Amatriciana

1. Set the skillet over medium heat again if needed. Add the spaghetti and about ½ cup / 120 ml pasta water, then toss for 1 to 2 minutes until the strands look evenly coated.
2. Take the skillet off the heat. Add the Pecorino Romano in 3 additions, tossing between each addition and loosening with small splashes of pasta water as needed until the sauce turns glossy and clings to the spaghetti instead of pooling at the bottom.
3. Taste and adjust with a little more cheese or chile if needed. Twirl into warm bowls and top with the reserved guanciale and extra Pecorino; success looks like shiny red strands with crisp-chewy bits throughout, not a dry pile and not a soupy puddle.