01 - Pat the chicken breasts dry with paper towels and season both sides generously with 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, reduce the heat to medium. Add the diced onion and cook for 2 to 3 minutes until softened and translucent. Stir in the minced garlic and cook for an additional 30 seconds until fragrant.
04 - Add the halved cherry tomatoes to the skillet and cook for approximately 3 minutes, allowing them 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 if using. Nestle the seared chicken breasts back into the skillet, spooning some of the sauce over the top.
07 - Cover the skillet with a lid and reduce the heat to low. Simmer gently for 12 to 15 minutes until the chicken reaches an internal temperature of 165°F (74°C) and is fully cooked through.
08 - Remove the lid and stir in the chopped spinach. Cook uncovered for 1 to 2 minutes until the leaves are just wilted and vibrant green.
09 - Taste the pan sauce and adjust the salt and pepper as needed. Serve immediately, garnished with freshly grated Parmesan cheese if desired.