Begin by boiling a large pot of salted water. Cook the giant pasta shells according to package instructions until al dente, usually about 8-10 minutes.

In a large mixing bowl, combine the shredded chicken, softened cream cheese, ranch dressing mix, half of the shredded cheddar cheese, sour cream, and optional spinach. Mix well until you have a creamy, uniform filling.

Using a spoon, carefully fill each cooked pasta shell with the chicken ranch mixture.

In a 9x13-inch baking dish, spread half of the marinara sauce on the bottom. Arrange the stuffed shells in a single layer on top of the sauce.

Pour the remaining marinara sauce over the stuffed shells. Sprinkle the remaining shredded cheddar cheese and Parmesan cheese on top, and season with salt and pepper to taste.

Preheat your oven to 350°F (175°C). Cover the baking dish with foil and bake for approximately 25 minutes.

Remove the foil, and bake for an additional 10 minutes, until the cheese is melted and bubbly.

Once baked, let the dish cool for a few minutes before digging in. Serve hot and enjoy with a fresh garden salad or some crusty bread.