Heat oil in a large heavy skillet over medium heat. Add guanciale and sauté until crisp and golden, about 4 minutes. Add pepper flakes and black pepper; stir for 10 seconds. Add onion and garlic; cook, stirring often, until soft, about 8 minutes. Add tomatoes, reduce heat to low, and cook, stirring occasionally, until sauce thickens, 15-20 minutes.
Meanwhile, bring a large pot of water to a boil. Season with salt; add the pasta and cook, stirring occasionally, until 2 minutes before al dente. Drain, reserving 1 cup of pasta cooking water.
Add drained pasta to sauce in skillet and toss vigorously with tongs to coat. Add 1/2 cup of the reserved pasta water and cook until sauce coats pasta and pasta is al dente, about 2 minutes. (Add a little pasta water if sauce is too dry.) Stir in cheese and transfer pasta to warmed bowls.
- 2 Tbsp. extra-virgin olive oil
- 4 oz. thinly sliced guanciale, pancetta, or chopped unsmoked bacon
- 1/2 tsp. crushed red pepper flakes
- 1/2 tsp. freshly ground black pepper
- 3/4 cup minced onion
- 2 cloves garlic, minced
- 1 28-oz. can peeled tomatoes with juices, crushed by hand t
- Kosher salt
- 12 oz. dried bucatini or spaghetti
- 1/4 cup finely grated Pecorino (about 1 oz.)