Preheat your oven to 375°F (190°C).
In a medium bowl, combine the chopped spinach, shredded Swiss cheese, garlic powder, onion powder, salt, and black pepper. Mix well until all ingredients are evenly incorporated.
Carefully lift the skin of each chicken thigh to create a pocket. Stuff each thigh with the spinach and cheese mixture, distributing it evenly among the thighs.
In a large oven-safe skillet, heat the olive oil over medium-high heat. Once hot, place the stuffed chicken thighs skin-side down in the skillet. Sear for about 5 minutes until the skin is golden brown.
Flip the chicken thighs over and sprinkle the breadcrumbs evenly on top. Transfer the skillet to the preheated oven and bake for 25-30 minutes, or until the chicken is cooked through and reaches an internal temperature of 165°F (74°C).
Remove the skillet from the oven and let the chicken rest for 5 minutes before serving. Garnish with chopped parsley if desired.