01 - Pat the chicken breasts dry with paper towels. Season both sides generously with salt and freshly ground black pepper.
02 - Heat the olive oil in a large skillet over medium-high heat. Add the chicken breasts and sear for 4 to 5 minutes per side until golden brown. Remove from the skillet and set aside.
03 - In the same skillet, add the diced onions and cook for 2 to 3 minutes until softened. Add the minced garlic and cook for another 30 seconds until fragrant.
04 - Add the halved cherry tomatoes to the skillet and cook for approximately 3 minutes, until they begin to 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 cook for 2 minutes to reduce slightly.
06 - Add the chicken broth, dried oregano, and crushed red pepper flakes if using. Return the seared chicken breasts to the skillet.
07 - Cover the skillet with a lid and simmer on low heat for 12 to 15 minutes, until the chicken reaches an internal temperature of 165°F (74°C).
08 - Stir in the chopped spinach and cook until just wilted, about 1 to 2 minutes.
09 - Taste the sauce and adjust seasonings if necessary. Serve hot, garnished with grated Parmesan cheese if desired.