Heat oven to 375°F. Spray 13x9-inch (3-quart) glass baking dish with cooking spray. In small bowl, mix spinach, onion and mozzarella cheese.
Between pieces of plastic wrap or waxed paper, place each chicken breast smooth side down; gently pound with flat side of meat mallet or rolling pin until about 1/8 inch thick. Sprinkle with garlic salt.
Place 1/4 of the spinach mixture on center of each flattened chicken breast. Fold in sides; fold up top and bottom, overlapping in center.
On shallow plate, mix bread crumbs and Parmesan cheese. Roll chicken in melted butter, then coat with bread crumb mixture. Place seam sides down in baking dish.
Bake 30 to 35 minutes, turning chicken over after 15 minutes, until light golden brown.