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 - Reduce the heat to medium. 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 about 2 minutes.
06 - Stir in the chicken broth, dried oregano, and red pepper flakes. Nestle the seared chicken breasts back into the skillet, spooning some of the sauce and vegetables over the top.
07 - Cover the skillet with a lid and reduce the heat to low. Gently simmer for 12 to 15 minutes until the chicken reaches an internal temperature of 165°F.
08 - Remove the lid and stir in the chopped spinach. Cook uncovered for 1 to 2 minutes, gently tossing until the leaves are just wilted and vibrant green.
09 - Taste the sauce and adjust the salt and pepper as needed. Serve immediately, garnished with freshly grated Parmesan cheese if desired.