Heat the 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 about 1 minute until fragrant.
Add the passata and crushed red pepper flakes, stir to combine, bring to a gentle simmer, then partially cover and simmer gently for 20–25 minutes, stirring occasionally.
While the sauce simmers, bring a generously salted pot of water to a boil and cook the pappardelle until al dente according to package directions.
Add the cooked chicken to the simmering sauce and warm through for a few minutes.
Season the sauce with salt and pepper to taste, then stir in the sliced or torn basil.
Reserve a splash of hot pasta water if needed, drain the pasta, and toss it with the sauce until evenly coated.
Serve topped with plenty of freshly grated Parmesan cheese.