Preheat oven to 375°F (190°C) and position the rack in the middle.
Butterfly each chicken breast by placing your hand flat on top and slicing from the thicker side toward the thinner side, stopping about 1/2 inch from the edge. Open the chicken like a book.
Cover the butterflied breasts with plastic wrap and gently pound to an even 1/4-inch thickness using the flat side of a meat mallet.
In a mixing bowl, combine crumbled feta, chopped spinach, minced garlic, dill, oregano, sun-dried tomatoes, lemon juice, and red pepper flakes. Mix thoroughly.
Mix salt, pepper, dried oregano, garlic powder, paprika, and lemon zest. Season both sides of each butterflied breast.
Divide the filling equally among the chicken breasts (approximately 2-3 tablespoons per breast), placing it on one half of each butterflied piece.
Fold the empty half over the filling, tucking in the sides as you roll. Secure with 3-4 toothpicks or kitchen twine.
Heat olive oil in an oven-safe skillet over medium-high heat. Once hot, add the stuffed chicken breasts and sear for 3-4 minutes per side until golden brown.
Transfer the skillet to the preheated oven and bake for 15-18 minutes, or until the internal temperature reaches 165°F (74°C).
Remove from oven, tent loosely with foil, and let rest for 5 minutes before slicing to allow juices to redistribute.