1. Prepare the Spinach Filling: In a medium bowl, combine the thawed and thoroughly squeezed dry spinach, softened cream cheese, shredded mozzarella, grated Parmesan cheese, minced garlic, onion powder, black pepper, and salt. Mix everything together until well combined and creamy. The mixture should be thick enough to hold its shape.

2. Prepare the Chicken Breasts: Pat the chicken breasts dry with paper towels. This helps with browning. Using a sharp knife, carefully slice a pocket into the thickest side of each chicken breast, being careful not to cut all the way through to the other side. You want to create a deep pocket for the filling.

3. Stuff the Chicken: Spoon an equal amount of the spinach and cheese mixture into each chicken pocket. Don’t overstuff, as the filling might ooze out during baking. If desired, secure the opening with a toothpick to keep the filling inside.

4. Season and Sear (Optional but Recommended): Heat the olive oil in a large oven-safe skillet over medium-high heat. Season the outside of the stuffed chicken breasts with a pinch more salt and pepper. Carefully place the chicken breasts into the hot skillet and sear for 2-3 minutes per side, until lightly golden brown. This step adds a delicious crust and helps seal in juices.

5. Bake to Perfection: Transfer the skillet with the chicken (or transfer chicken to a baking dish if your skillet isn’t oven-safe) to a preheated oven at 375 F (190 C). Bake for 20-25 minutes, or until the internal temperature of the chicken reaches 165 F (74 C) when measured with a meat thermometer at the thickest part. The chicken should be opaque throughout, and the filling should be bubbly.

6. Rest and Serve: Once cooked, remove the chicken from the oven and let it rest for 5 minutes before slicing and serving. This allows the juices to redistribute, resulting in more tender and flavorful chicken. Garnish with fresh parsley if desired.