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 the chicken 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 about 3 minutes, stirring occasionally, until they begin to soften and release their 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 to concentrate the flavors.
06 - Stir in the chicken broth, dried oregano, and crushed red pepper flakes. 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. Avoid lifting the lid frequently to maintain consistent heat.
08 - Remove the lid and scatter the chopped spinach over and around the chicken. Stir gently and cook for 1 to 2 minutes until the leaves are just wilted and bright green.
09 - Taste the pan sauce and adjust the salt and pepper as needed. Serve the chicken hot, spooned with the tomato-spinach sauce, and garnished with grated Parmesan cheese if desired.