01 - Pat the chicken breasts dry with paper towels. Season both sides generously with kosher salt and freshly cracked black pepper.
02 - Heat the olive oil in a large skillet over medium-high heat. Place the chicken breasts in the skillet and sear for 3 to 4 minutes per side until a golden crust forms. Transfer the chicken to a plate and set aside.
03 - Reduce the heat to medium. In the same skillet, add the chopped onion and sauté until softened and translucent, about 3 minutes. Add the minced garlic and stir continuously until fragrant, approximately 30 seconds.
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 to meld the flavors.
05 - Return the seared chicken breasts to the skillet, nestling them into the tomato sauce. Cover with the 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 grated Parmesan cheese evenly over each chicken breast. Continue cooking uncovered for 2 minutes to allow the cheese to melt and the sauce to slightly thicken.
07 - Scatter the torn fresh basil over the top of the dish. Serve hot, spooning extra sauce over each portion, with additional Parmesan on the side.