01 - Combine ricotta cheese, shredded mozzarella cheese, cooked spinach, egg, salt, and pepper in a mixing bowl. Mix until uniformly blended.
02 - Fill each cooked pasta shell generously with the spinach and cheese mixture.
03 - Arrange the stuffed shells in a single layer in a baking dish. Pour marinara sauce evenly over the shells.
04 - Cover the dish tightly with foil and freeze until needed. For baking, preheat oven to 375°F (190°C) and bake the frozen shells covered for 45 to 50 minutes until heated through and bubbling.