Preheat the oven to 400°F (200°C).

Heat oil in a skillet over medium heat. Add the spinach and cook for 3-4 minutes, until wilted. Season with salt, pepper, and garlic powder. Stir in the feta and sun-dried tomatoes, then remove from heat and set aside.

Butterfly the chicken by slicing it horizontally, being careful not to cut all the way through. You should be able to open it flat in one piece. Using a wooden mallet or rolling pin, pound the chicken to flatten it slightly. Season both sides with paprika, salt, and pepper.

Spoon the spinach mixture onto one side of each chicken breast, spreading it evenly. Fold the chicken over to enclose the filling. Seal the edges with toothpicks so that there are no holes for the filling to escape.

Heat the remaining oil in the skillet over medium-high heat. Add the chicken and sear for 3-4 minutes per side, until browned but not fully cooked.

Transfer the chicken to a baking dish (if not using an oven-safe skillet) and bake uncovered for 15-18 minutes, until the chicken is fully cooked. Remove toothpicks and serve.