Preheat the oven to 375°F (190°C). Spray the baking dish with cooking spray and set aside.
In a mixing bowl, combine the cooked chicken and marinara sauce, mixing well. Spread this mixture evenly in the bottom of the prepared baking dish.
In another bowl, whisk together the eggs, garlic powder, Italian seasoning, salt, and black pepper.
Dip each piece of the chicken into the egg mixture, then coat with breadcrumbs, pressing lightly to adhere.
Heat the olive oil in a large skillet over medium heat. Add the breaded chicken pieces and cook for about 3-4 minutes on each side until golden brown.
Once the chicken is cooked, slice it into strips and layer it over the marinara sauce in the baking dish.
Sprinkle the shredded mozzarella cheese and grated Parmesan cheese evenly on top of the layered chicken.
Cover the baking dish loosely with aluminum foil and bake in the preheated oven for 30 minutes.
After 30 minutes, remove the foil and cook for an additional 10 minutes, allowing the cheese to melt and turn golden brown.
Once done, remove from the oven and let it cool for about 5 minutes before serving.