Preheat the oven to 375°F.
Place the chicken breasts between two pieces of plastic wrap and pound them to an even thickness, about 1/2 inch thick.
In a bowl, mix the chopped spinach, feta cheese, sun-dried tomatoes, garlic powder, onion powder, salt, black pepper, and red pepper flakes (if using) until well combined.
Lay the chicken breasts flat on a clean surface. Divide the spinach and feta mixture evenly among the chicken breasts, spreading it out to cover the surface.
Starting from one end, carefully roll each chicken breast tightly around the filling. Secure the ends with toothpicks or tie with kitchen twine to hold the filling inside.
In a large oven-safe skillet, heat the olive oil over medium-high heat. Once hot, add the rolled chicken and sear for about 3-4 minutes on each side until golden brown.
Drizzle the balsamic vinegar over the chicken, then transfer the skillet to the preheated oven. Bake for 20-25 minutes or until the chicken is cooked through (internal temperature should reach 165°F).
Remove the chicken from the oven and let it rest for 5 minutes before slicing. Serve warm.