Season the chicken breasts with salt, pepper and Italian seasoning. Fold the short end of the chicken breast under the larger end to make the chicken breast into a rounded shape rather than oblong and secure with a toothpick. Pre-heat a skillet over medium-high heat. Add the olive oil and brown the chicken breasts on both sides. Remove the toothpicks and set the chicken breasts aside to cool.

Preheat oven to 425°F.

Squeeze the liquid out of the defrosted frozen spinach. Combine the softened butter and cream cheese in a bowl and fold in the Parmesan cheese, Monterey Jack cheese, spinach, artichoke hearts and garlic powder. Season with salt and pepper.

Roll out puff pastry to a 12-inch by 12-inch square. Cut the pastry into 4 equal 6-inch squares. Spread a quarter of the spinach mixture in the center of each square of puff pastry. Place a chicken breast on top of each pile of spinach. Fold the puff pastry up around the chicken, pinching the seams shut. You may have to gently stretch the dough to cover the chicken breast. Place the seam side down on a baking sheet and brush the outside of the dough with the egg wash. Bake for 25 to 30 minutes, until puff pastry is browned and chicken reaches an internal temperature of 165°F.