Preheat your oven to 400°F (200°C). In a medium mixing bowl, combine the softened cream cheese, crumbled feta, finely chopped spinach, minced garlic, and optional red pepper flakes. Mix until well combined. Season with a pinch of salt and pepper if needed (feta is already salty).

Pat the chicken breasts dry with a paper towel. Place a chicken breast on a cutting board, and with your hand flat on top, carefully slice horizontally into the thickest side to create a pocket. Be careful not to cut all the way through.

In a small bowl, mix together the paprika, garlic powder, salt, and black pepper. Season the entire outside of each chicken breast with this mixture.

Carefully spoon about a quarter of the spinach and feta filling into the pocket of each chicken breast. Don't overstuff. Secure the opening with 2-3 toothpicks to keep the filling from leaking out during cooking.

Heat the olive oil in a large, oven-safe skillet over medium-high heat. Once the oil is shimmering, carefully place the stuffed chicken breasts in the skillet.

Sear the chicken for 3-4 minutes on each side, until a beautiful golden-brown crust forms. This step is crucial for flavor and a juicy result.

Transfer the entire skillet to the preheated oven. Bake for 15-20 minutes, or until the chicken is cooked through and the internal temperature reaches 165°F (74°C) on an instant-read thermometer.

Remove the skillet from the oven and let the chicken rest in the pan for 5-10 minutes. This allows the juices to redistribute. Remember to remove the toothpicks before slicing and serving.