Preheat your oven to 375°F.
Place the chicken breasts between two sheets of plastic wrap and pound them to an even thickness, about 1/2 inch thick.
In a bowl, combine the chopped spinach, feta cheese, sun-dried tomatoes, garlic powder, onion powder, salt, black pepper, and red pepper flakes (if using). Mix well.
Lay the chicken breasts flat on a clean surface and evenly distribute the spinach and feta mixture over each breast.
Starting from one end, carefully roll up each chicken breast tightly and secure with toothpicks or kitchen twine.
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 and sprinkle with dried oregano.
Transfer the skillet to the preheated oven and bake for 20-25 minutes, or until the chicken is cooked through (internal temperature of 165°F).
Remove from the oven, let rest for 5 minutes, then remove toothpicks or twine before slicing.