Instructions

• Preheat the oven to 350°F (175°C). Spray a 9×13-inch baking dish with cooking spray and set aside.
• Bring a large pot of salted water to a boil. Add the 24 jumbo shells and cook according to package directions until al dente (about 10–12 minutes). Drain, rinse shells with cold water to stop cooking, drizzle with a little of the 1 tablespoon olive oil to prevent sticking, and set aside.
• While the shells are cooking, heat 1 tablespoon olive oil in a large skillet over medium heat. Add 2 to 3 cloves garlic, finely minced, and the 4 cups fresh spinach. Sauté, stirring frequently, until the spinach is wilted and the garlic is fragrant, about 3–4 minutes.
• Remove the skillet from the heat and let the spinach mixture cool on the stove for about 5 minutes. Transfer the spinach to a few layers of paper towels, fold the towels around the spinach, and squeeze firmly to remove as much excess moisture as possible.
• Place the squeezed spinach on a cutting board and finely chop. Transfer the chopped spinach to a large mixing bowl.
• To the bowl with the spinach, add 2 cups ricotta cheese, 1/2 cup shredded Parmesan cheese, 1 egg, 1 teaspoon salt, 1/2 teaspoon ground pepper, 1/2 teaspoon dried parsley, 1/2 teaspoon dried basil, and 1/2 teaspoon dried oregano. Stir until the mixture is well combined and uniform in texture. Taste and adjust seasoning if desired.
• Spoon about 3 to 4 tablespoons of the 2 1/2 cups jarred marinara sauce into the bottom of the prepared baking dish and spread into a thin layer.
• Using about 1 1/2 tablespoons of the ricotta-spinach mixture per shell, fill each cooked jumbo shell. Place each filled shell in the prepared baking dish with the open side up, arranging them in a single layer.
• Pour the remaining marinara sauce (the rest of the 2 1/2 cups jarred marinara) evenly over the filled shells so they are mostly covered.
• Sprinkle 1 cup shredded mozzarella cheese evenly over the sauced shells.
• If baking immediately: place the dish on the center oven rack and bake for about 30 minutes, rotating the pan once halfway through baking, until the edges are bubbling and the cheese is lightly golden.
• If you assembled the dish ahead of time: cover tightly with foil and refrigerate for up to 24–48 hours. If refrigerated, remove the covered dish and let it sit at room temperature for 30–45 minutes before baking to avoid extra baking time from a cold dish. If frozen, thaw thoroughly before baking.
• To freeze assembled shells: you may freeze the fully assembled casserole (covered) or freeze just the stuffed shells (without sauce and mozzarella) in a large zip-top freezer bag; make sure the container or bag is sealed well.
• Garnish with fresh parsley if desired before serving. Store leftovers airtight in the refrigerator for up to 5 days or in the freezer for up to 3 months; reheat gently as needed.