Put the guanciale in a cold, deep skillet. Set it over medium-low heat and cook for 8–10 minutes, stirring occasionally, until the fat has rendered and the guanciale is golden and crisp. Transfer it to a plate with a slotted spoon. Leave about 2 tablespoons of fat in the skillet. If you have less, add a little olive oil as needed.

Add the onion and cook over medium-low heat for about 5 minutes, stirring occasionally, until soft. Stir in 1 teaspoon of Calabrian pepper and cook for 30 seconds. Add the squash and ½ cup water. Cover and simmer for 15–20 minutes, stirring once or twice, until the squash is very tender. Add a splash of water if the pan starts to dry out.

While the squash cooks, bring a large pot of lightly salted water to a boil. Cook the fusilloni until 1 minute shy of al dente according to the package directions. Reserve 1 cup of pasta water, then drain.

Take the skillet off the heat and blend the squash mixture until smooth using an immersion blender. Return the skillet to medium-low heat, add the pasta and ¼ cup reserved pasta water, and toss for 1–2 minutes until the pasta is al dente and coated. Add more pasta water, a tablespoon at a time, if the sauce is too thick.

Turn off the heat and stir in the Parmigiano. Taste and add salt or more Calabrian pepper as needed. Divide among four bowls and top with the crispy guanciale. Serve immediately.