In a medium bowl, mix together the chopped spinach, feta cheese, cream cheese, minced garlic, red pepper flakes (if using), oregano, lemon zest, and lemon juice. Season the mixture with salt and pepper to taste.
Carefully slice a pocket into each chicken breast by cutting horizontally through the thickest part, making sure not to cut all the way through.
Stuff each chicken breast with the spinach and feta mixture, ensuring it's well-filled. Use toothpicks to secure the openings if necessary.
Heat olive oil in a large oven-safe skillet over medium-high heat. Season the chicken breasts with salt and pepper, then sear them in the skillet for about 4-5 minutes on each side, until golden brown.
Once the chicken is seared, transfer the skillet to the preheated oven and bake for 20-25 minutes, or until the chicken is cooked through (internal temperature should reach 165°F or 75°C).
Remove the chicken from the oven and let it rest for a few minutes before serving.
Notes
Serve the stuffed chicken breasts on a bed of sautéed vegetables or alongside roasted potatoes. Drizzle with a bit of extra lemon juice for brightness and garnish with fresh herbs for added color.