Follow these steps carefully for perfectly stuffed, juicy chicken:

1. Preheat and Prepare: Preheat your oven to 400°F (200°C). Pat the chicken breasts completely dry with paper towels – this is crucial for getting a good sear later.
2. Sauté Aromatics and Spinach: Heat 1 tablespoon of olive oil in a large skillet over medium heat. Add the minced garlic and sauté for about 30 seconds until fragrant, being careful not to burn it. Add the chopped fresh spinach to the skillet. It will seem like a lot, but it will wilt down significantly. Cook, stirring occasionally, until the spinach is fully wilted and most of the moisture has evaporated (about 3-5 minutes). Squeezing out excess moisture is key to prevent a watery filling. Remove from heat and let it cool slightly.
3. Make the Filling: In a medium bowl, combine the slightly cooled sautéed spinach and garlic mixture, crumbled feta cheese, softened cream cheese, black pepper, salt (start with less, you can adjust), and optional oregano/Italian seasoning and red pepper flakes. Mix thoroughly until well combined. The cream cheese should be evenly distributed, creating a cohesive filling. Taste and adjust seasoning if necessary.
4. Prepare the Chicken Breasts: Place a chicken breast flat on a cutting board. Carefully slice a deep pocket into the thickest side of the breast, parallel to the cutting board. Cut horizontally, stopping about ½ inch from the other side, creating a flap. Be careful not to cut all the way through. Repeat with the remaining chicken breasts. If your chicken breasts are very thick, you can gently pound the thicker side before cutting the pocket to ensure more even cooking, but be gentle not to tear the meat.
5. Stuff the Chicken: Divide the spinach and feta filling evenly among the four chicken breasts, carefully spooning it into the pockets you created. Don’t overstuff, as the filling might leak out during cooking. Gently press the edges of the pocket together.
6. Secure (Optional): If needed, secure the opening of each chicken breast with one or two toothpicks to help keep the filling inside during searing and baking. This is especially helpful if the pockets are quite full.
7. Sear the Chicken (Highly Recommended): If you used an oven-safe skillet for the spinach, you can wipe it out and reuse it. Otherwise, heat another tablespoon of olive oil (or use the same skillet if large enough and oven-safe) over medium-high heat. Once the oil is shimmering hot, carefully place the stuffed chicken breasts in the skillet, presentation side down (the smoother side). Sear for 2-3 minutes per side, until golden brown. Searing develops flavor (thanks to the Maillard reaction) and creates an appealing crust. Don’t overcrowd the pan; sear in batches if necessary.
8. Bake the Chicken: If your skillet is oven-safe, transfer it directly to the preheated oven. If not, transfer the seared chicken breasts to a lightly greased baking dish. Bake at 400°F (200°C) for 20-25 minutes, or until the chicken is cooked through. The internal temperature should reach 165°F (74°C) when checked with a meat thermometer inserted into the thickest part of the chicken (not the filling). Cooking time will vary depending on the thickness of the chicken breasts.
9. Rest the Chicken: Once cooked, remove the chicken from the oven (or skillet). Let it rest on a cutting board or serving platter for 5-10 minutes before slicing. This crucial step allows the juices to redistribute throughout the meat, ensuring a tender and juicy result. If you skip resting, the juices will run out when you slice, leaving you with drier chicken.
10. Serve: Remove any toothpicks before serving. Slice the chicken breasts crosswise (either in half or into medallions) to showcase the beautiful filling, or serve them whole.