Cook and drain the shells according to the package directions. While the shells are cooking, heat the oven to 400°F.
Heat the oil in a 12-inch skillet over medium-high heat. Add the onion and cook for 2 minutes. Add the mushrooms and cook for 2 minutes or until tender. Season with salt and pepper.
Stir the ricotta, egg, 1/4 cup soup, 2 tablespoons Parmesan cheese and 1/2 cup mozzarella cheese in a bowl. Stir in the mushroom mixture, chicken and spinach.
Stir the remaining soup, 2 tablespoons Parmesan cheese and the stock in another bowl. Spread about 1/2 cup soup mixture in the bottom of a 13x9x2-inch baking dish.
Spoon about 2 tablespoons chicken mixture into each shell. Arrange the stuffed shells in the baking dish. Pour the remaining soup mixture over the stuffed shells. Sprinkle with the remaining mozzarella cheese. Cover the baking dish.
Bake for 20 minutes. Uncover and bake for another 10 minutes or until hot.
This nutritional information refers to each serving of the entire recipe and not just the products used as ingredients.