Microwave spinach in a microwave-safe bowl until defrosted, about 5 minutes. Drain.
Place chicken thighs in a gallon-sized resealable bag and pound with a heavy glass bottle until about 1/4-inch in thickness. Pat dry with paper towels.
Combine the warm spinach, mozzarella cheese, Parmesan cheese, sun-dried tomatoes, garlic powder, salt, and pepper in a bowl and mix until cheeses melt. Spread over the chicken thighs; roll and secure with toothpicks.
Heat olive oil in an oven-safe skillet over medium-high heat. Place the chicken thighs seam-side down and cook until browned, 3 to 4 minutes per side.
Transfer skillet to the preheated oven and bake the chicken until no longer pink in the center and the juices run clear, 20 to 25 minutes.