Preheat your oven to 375°F (190°C).
In a mixing bowl, combine the chopped spinach, Swiss cheese, cream cheese, minced garlic, onion powder, paprika, salt, and pepper. Mix until well combined.
Carefully slice a pocket into each chicken thigh, being careful not to cut all the way through.
Stuff each chicken thigh with the spinach and cheese mixture, securing the opening with toothpicks if necessary.
Heat olive oil in an oven-safe skillet over medium-high heat. Once hot, add the stuffed chicken thighs and sear for about 3-4 minutes on each side until golden brown.
Transfer the skillet to the preheated oven and bake for 20-25 minutes, or until the chicken is cooked through and reaches an internal temperature of 165°F (74°C).
Remove from the oven and let rest for 5 minutes. Garnish with fresh parsley before serving.