In a medium bowl, combine the chopped spinach, chopped artichoke hearts, softened cream cheese, shredded mozzarella, grated Parmesan, minced garlic, onion powder, garlic powder, salt, and pepper. Mix well until evenly distributed and creamy.
Place each chicken breast on a cutting board. Using a sharp knife, carefully slice horizontally through the thickest part of the breast, stopping about a half-inch from the edge so it opens like a book. Be gentle to avoid cutting all the way through.
Evenly divide the spinach-artichoke mixture among the chicken breasts, placing it inside each butterflied pocket. Fold the chicken back over to enclose the filling. Secure with toothpicks if needed.
Heat olive oil in a large skillet over medium-high heat. Once hot, add the stuffed chicken breasts. Sear for 3-4 minutes on each side until golden brown.
Transfer the seared chicken breasts to a baking dish. Bake in a preheated oven at 375°F (190°C) for 20-25 minutes, or until the internal temperature reaches 165°F (74°C).
Remove the chicken from the oven and tent loosely with foil. Let it rest for 5 minutes before serving to keep it moist.