Heat 1/4 cup olive oil in a large pot, braiser, or deep skillet over medium heat.
Add the chopped onion and sauté for 4–5 minutes until softened and translucent.
Stir in the minced garlic and cook for 1 minute until fragrant.
Pour in the 24 ounces of passata and add 1 teaspoon crushed red pepper flakes; stir to combine.
Bring the sauce to a gentle simmer, partially cover the pot, and simmer over low heat for 20–25 minutes, stirring occasionally.
Meanwhile, bring a large pot of generously salted water to a boil and cook the pappardelle according to package directions until al dente.
Add the 2 cups cooked chicken to the simmering sauce and warm through for a few minutes.
Season the sauce with salt and freshly ground black pepper to taste, then stir in the sliced or torn basil.
Drain the pasta, reserving a splash of hot pasta water if needed to thin the sauce, then toss the pasta with the sauce until well coated.
Serve topped with freshly grated Parmesan cheese.
