Preheat oven to 375˚F. Lightly coat a baking dish with cooking spray and set aside.
Slightly pound each chicken breast to an even thickness - DO NOT pound it thin. Cut a pocket on the side of each chicken breast and carefully cut it open like a book - do not cut through. Season with salt and pepper and set aside.
Heat olive oil in a nonstick skillet. Add spinach and garlic; season with a pinch of salt and cook for 2 minutes, or until wilted, stirring frequently.
Remove from heat and let the spinach cool down.
When cool enough, combine the spinach, cottage cheese, 1/2 cup mozzarella cheese, egg white, salt, and pepper in a mixing bowl; mix and stir until well combined.
Evenly spoon the spinach mixture over the chicken breasts.
Seal the chicken breasts with a toothpick and transfer to the baking dish.
Combine the spaghetti sauce and tomato paste in a small mixing bowl; stir until well combined. Spoon sauce mixture over the chicken pieces.
Cover the dish and bake for 25 minutes.
Uncover and sprinkle with remaining mozzarella cheese. Put back in the oven and continue to cook for 20 more minutes, or until cheese is melted and chicken is cooked through.