Preheat the oven to 375°F (190°C). Lightly grease a baking dish or line it with parchment paper.
Flatten the chicken breasts between parchment or plastic wrap using a mallet or rolling pin until about ½-inch thick.
Make the filling by combining chopped spinach, sun-dried tomatoes, crumbled feta, and optional mozzarella in a bowl.
Season the chicken on both sides with garlic powder, onion powder, oregano, salt, and pepper.
Add the filling down the center of each breast. Roll tightly and secure with toothpicks or kitchen twine.
Sear the chicken rolls in olive oil over medium-high heat for 2–3 minutes per side until golden brown.
Bake the rolls in the oven for 20–25 minutes, or until the internal temperature reaches 165°F (74°C).
Rest and serve. Let the chicken rest for a few minutes, then remove toothpicks, slice if desired, and garnish with herbs.