01 - Pat the chicken breasts dry with paper towels and season both sides generously with salt and freshly ground black pepper.
02 - Heat olive oil in a large skillet over medium-high heat. Add the chicken breasts and sear for 4 to 5 minutes per side until a golden-brown crust forms. Remove from the skillet and set aside on a plate.
03 - In the same skillet, add the diced onion and cook for 2 to 3 minutes until softened. Add the minced garlic and stir for 30 seconds until fragrant.
04 - Add the halved cherry tomatoes to the skillet and cook for 3 minutes until they begin to soften and release their juices.
05 - Pour in the dry white wine, scraping up any browned bits from the bottom of the skillet. Bring to a simmer and let reduce for 2 minutes.
06 - Stir in the chicken broth, dried oregano, and red pepper flakes if using. Return the seared chicken breasts to the skillet, nestling them into the sauce.
07 - Cover the skillet with a lid and reduce heat to low. Simmer for 12 to 15 minutes until the chicken reaches an internal temperature of 165°F (74°C).
08 - Remove the lid, stir in the chopped spinach, and cook for 1 to 2 minutes until just wilted.
09 - Taste the sauce and adjust salt and pepper as needed. Serve hot, garnished with grated Parmesan cheese if desired.