Preheat the oven to 375°F / 190°C.
Lightly coat a 9x13-inch rectangular baking dish with cooking spray.
In a medium mixing bowl, combine the sour cream, 1/2 cup grated Parmesan cheese, cornstarch, dried oregano, dried basil, garlic powder, salt, and black pepper.
Stir until the mixture is smooth and evenly combined.
Arrange the boneless, skinless chicken breasts in a single layer in the prepared baking dish.
Place one slice of mozzarella cheese on top of each chicken breast.
Spoon the sour cream mixture evenly over the chicken and mozzarella.
Spread the mixture gently to fully cover each chicken breast.
Sprinkle the remaining 1/4 cup grated Parmesan cheese evenly over the top.
Bake uncovered for 45 minutes to 1 hour, depending on the thickness of the chicken breasts.
Check that the chicken is fully cooked before serving. The internal temperature should reach 165°F / 74°C in the thickest part of each chicken breast.
Remove the baking dish from the oven and allow the chicken to rest for several minutes.
Garnish with chopped parsley.
Serve warm over rice, spooning the creamy sauce over the chicken and rice before serving.