01 - Pat chicken breasts dry with paper towels and season both sides generously with salt and black pepper.
02 - Heat olive oil in a large skillet over medium-high heat. Add chicken breasts and sear for 3 to 4 minutes per side until a golden crust forms. Remove from skillet and set aside on a plate.
03 - Reduce 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.
04 - Add the crushed tomatoes, fresh diced tomatoes, dried oregano, and red pepper flakes to the skillet. Stir to combine and 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 for 15 to 18 minutes, or until the chicken reaches an internal temperature of 165°F.
06 - Uncover the skillet and sprinkle the chicken with grated Parmesan cheese. Continue cooking uncovered for 2 minutes to allow the cheese to melt and the sauce to thicken slightly.
07 - Scatter torn fresh basil leaves over the top. Serve hot, spooning extra sauce over each chicken breast, and pass additional Parmesan at the table.