01 - Pat the chicken breasts dry with paper towels and season both sides generously with kosher salt and freshly ground black pepper.
02 - Heat the olive oil in a large skillet over medium-high heat. Add the chicken breasts and sear for 3 to 4 minutes per side until a golden crust forms. Remove the chicken from the skillet and set it aside on a plate.
03 - Reduce the heat to medium. In the same skillet, add the chopped onion and sauté until translucent, about 3 minutes. Stir in the minced garlic and cook until fragrant, about 30 seconds more.
04 - Pour in the crushed tomatoes along with the fresh diced tomatoes, dried oregano, and red pepper flakes. Stir to combine and let the sauce simmer for 5 minutes, stirring occasionally.
05 - Return the seared chicken breasts to the skillet, nestling them into the sauce. Cover with a lid and simmer gently for 15 to 18 minutes, or until the chicken reaches an internal temperature of 165°F and is cooked through.
06 - Remove the lid and sprinkle the Parmesan cheese evenly over the chicken breasts. Continue cooking uncovered for 2 minutes until the cheese melts and the sauce thickens slightly.
07 - Scatter the torn fresh basil over the top. Serve hot, spooning extra sauce over each portion, with additional Parmesan on the side.