Add the olive oil to a pot, braiser, or deep skillet and sauté the onion for 4-5 minutes over medium heat.
Stir in the garlic and cook for 1 minute.
Add in the passata and crushed red pepper flakes and give it a good stir. Once it starts to bubble, cover the pot with the lid slightly open and simmer over low heat for 20-25 minutes, stirring occasionally (it should be bubbling gently).
Meanwhile, boil a generously salted pot of water for the pappardelle. Cook it al dente according to package directions.
Add the chicken to the sauce and let it warm through for a few minutes.
Season the sauce with salt & pepper (I am fairly generous with the salt) and stir in the fresh basil. Drain the pasta and toss until coated (add a splash of the hot pasta water prior to draining it to thin the sauce if needed). Serve with plenty of freshly grated parmesan cheese.
