01 - Preheat your oven to 400°F. Lightly grease a baking dish with cooking spray or a small amount of olive oil.
02 - Slice each chicken breast lengthwise through the thickest part, opening it like a book without cutting completely through. Season both sides with 1/2 teaspoon salt and 1/2 teaspoon black pepper.
03 - Combine ricotta cheese, chopped spinach, 1/3 cup parmesan, mozzarella, minced garlic, nutmeg, 1/4 teaspoon salt, and 1/4 teaspoon pepper in a mixing bowl. Mix until thoroughly incorporated.
04 - Divide the spinach ricotta mixture evenly among the chicken breasts, spreading it over one side of each butterflied breast. Fold the opposite side over to enclose the filling completely.
05 - Secure the stuffed breasts with toothpicks if needed to prevent filling from escaping during baking. Arrange in the prepared baking dish. Brush tops with 1 tablespoon olive oil and sprinkle with remaining 1/4 cup parmesan cheese.
06 - Bake for 25-30 minutes until the chicken reaches an internal temperature of 165°F and the tops develop a golden brown crust. The filling should be hot and bubbly.
07 - Remove from oven and let the chicken rest for 5 minutes to allow juices to redistribute. Remove toothpicks carefully. Slice crosswise to reveal the filling and serve immediately.