Step 1: Preheat your oven to 375°F (190°C). Pour one jar (24-28 oz) of your favorite marinara sauce into a 9x13 inch baking dish, spreading it evenly across the bottom.
Step 2: Bring a large pot of salted water to a rolling boil. Add 24-30 large pasta shells and cook according to package directions until al dente (8-10 minutes). Drain, rinse with cold water, and lay out to cool slightly.
Step 3: In a large mixing bowl, combine 15 oz whole milk ricotta cheese, 10 oz thawed and well-drained frozen chopped spinach, 1 large egg, ½ cup grated Parmesan cheese, ¼ cup chopped fresh parsley, 1 teaspoon garlic powder, ½ teaspoon onion powder, ½ teaspoon salt, and ¼ teaspoon black pepper.
Step 4: Mix all the filling ingredients thoroughly until well combined and taste to adjust seasonings if necessary.
Step 5: Carefully fill each cooked pasta shell with approximately 1-2 tablespoons of the ricotta-spinach mixture. Arrange the stuffed shells in a single layer over the marinara sauce in the prepared baking dish.
Step 6: Pour the remaining jar (24-28 oz) of marinara sauce over the top, ensuring all shells are covered.
Step 7: Sprinkle 1 ½ cups of shredded mozzarella cheese evenly over the sauced shells.
Step 8: Cover the baking dish tightly with aluminum foil. Bake for 25 minutes.
Step 9: Remove the foil and continue to bake for another 10-15 minutes, or until the cheese is melted, bubbly, and lightly golden brown. Optionally, broil for 1-2 minutes for a crispier top.
Step 10: Carefully remove the dish from the oven. Let the stuffed shells rest for 5-10 minutes before serving. Garnish with fresh parsley or basil, and serve hot!