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. Once the oil shimmers, add the chicken breasts and sear for 4 to 5 minutes per side until a deep golden crust forms. Transfer to a plate and set aside.
03 - In the same skillet, add the diced onion and cook for 2 to 3 minutes until softened and translucent. Stir in the minced garlic and cook for 30 seconds until fragrant.
04 - Add the halved cherry tomatoes to the skillet and cook for approximately 3 minutes, stirring occasionally, until they begin to soften and release their natural juices.
05 - Pour in the dry white wine, using a wooden spoon to scrape up any browned bits from the bottom of the skillet. Bring the liquid to a simmer and let it reduce for 2 minutes.
06 - Stir in the chicken broth, dried oregano, and red pepper flakes. Return the seared chicken breasts to the skillet, nestling them into the sauce.
07 - Cover the skillet with a lid, reduce the heat to low, and simmer for 12 to 15 minutes until the chicken reaches an internal temperature of 165°F.
08 - Remove the lid, stir in the chopped spinach, and cook uncovered for 1 to 2 minutes until the leaves are just wilted and vibrant green.
09 - Taste the sauce and adjust the salt and pepper as needed. Serve hot, garnished with freshly grated Parmesan cheese if desired.