Preheat your oven to 375°F.
In a mixing bowl, combine the chopped spinach, Swiss cheese, cream cheese, minced garlic, onion powder, paprika, salt, and pepper. Mix until well combined.
Using a sharp knife, carefully create a pocket in each chicken thigh by slicing along one side, being careful not to cut all the way through.
Stuff each chicken thigh with the spinach and cheese mixture, using toothpicks to secure the opening if necessary.
Heat the olive oil in a large oven-safe skillet over medium-high heat. Once hot, add the stuffed chicken thighs, skin-side down, and sear for about 5 minutes until the skin is golden brown.
Flip the chicken thighs over and transfer the skillet to the preheated oven. Bake for 25-30 minutes, or until the chicken reaches an internal temperature of 165°F.
Remove from the oven and let rest for 5 minutes before serving.