Microwave spinach in a medium bowl until warmed through, 2 to 3 minutes. Let cool slightly and squeeze to remove excess moisture.

Return spinach to the bowl. Mix in cream cheese, artichoke hearts, Parmesan cheese, mayonnaise, garlic powder, and salt. Set aside.

Pound chicken breasts to an even thickness, no more than 1-inch-thick. Season with salt and pepper on both sides.

Preheat the oven to 375 degrees F (190 degrees C).

Heat olive oil in a large skillet over medium-high heat. Brown chicken breasts in the hot oil, 2 to 3 minutes, working in batches if necessary. Place chicken breasts in a large baking dish. Spread spinach-artichoke mixture on top of each breast.

Bake in the preheated oven until chicken is no longer pink in the center and the juices run clear, 20 to 22 minutes. An instant-read thermometer inserted into the center should read at least 165 degrees F (74 degrees C). Top breasts with the mozzarella cheese and continue baking until cheese has melted, 1 to 2 more minutes.