Preheat your oven to 375°F (190°C).
Using a meat mallet, gently pound each chicken breast to an even thickness of about ½ inch.
In a mixing bowl, combine the chopped spinach, cream cheese, shredded mozzarella, garlic powder, onion powder, Italian herbs, and seasoning (salt and pepper). Mix well until all ingredients are combined.
Take one chicken breast and place a portion of the stuffing mixture in the center. Fold the chicken over to enclose the filling, and secure the edges with toothpicks or tie with kitchen twine. Repeat with the remaining chicken breasts.
Heat olive oil in a frying pan over medium-high heat. Once hot, sear each stuffed chicken breast for 3-4 minutes on each side, until golden brown.
Transfer the seared stuffed chicken breasts to a baking dish and cover with aluminum foil. Bake in the preheated oven for 20-25 minutes, or until the chicken reaches an internal temperature of 165°F (75°C).
Remove from the oven and let rest for a few minutes before removing toothpicks or twine. Slice and serve with your favorite side dish.