01 - Pat the chicken breasts thoroughly dry with paper towels. 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 until shimmering. Carefully lay the chicken breasts in the pan and sear for 3 to 4 minutes per side until a deep 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 soft and translucent, about 3 minutes. Add the minced garlic and stir constantly for 30 seconds until fragrant.
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 gently for 5 minutes, stirring occasionally to meld the flavors.
05 - Nestle the seared chicken breasts back into the skillet, spooning sauce over the top. Cover with a lid and simmer over medium-low heat for 15 to 18 minutes, until the chicken reaches an internal temperature of 165°F and is fully cooked through.
06 - Remove the lid and sprinkle the grated Parmesan evenly over each chicken breast. Continue cooking uncovered for 2 minutes, allowing the cheese to melt and the sauce to thicken slightly.
07 - Scatter the torn fresh basil leaves over the top. Serve immediately while hot, spooning extra sauce over each portion and passing additional Parmesan at the table.