Follow these step-by-step instructions carefully to ensure your Spinach Ricotta Stuffed Shells come out perfectly every time. The process is straightforward and can be broken down into a few key stages: preparing the components, assembling the dish, and baking to perfection.

Step 1: Cook the Pasta Shells
Bring a large pot of heavily salted water to a rolling boil. Add the entire 12-ounce box of jumbo pasta shells and cook according to the package directions, aiming for al dente. The shells should be tender but still have a firm bite. It’s crucial not to overcook them, as they will continue to cook in the oven and can become mushy if boiled for too long. Once cooked, drain the shells carefully in a colander and rinse them with cold water. This immediately stops the cooking process and prevents them from sticking together. Set them aside while you prepare the filling.

Step 2: Prepare the Spinach
While the pasta is cooking, prepare your spinach. If you haven’t already, thaw the frozen chopped spinach completely. The most effective way to remove excess water is to place the thawed spinach in the center of a clean kitchen towel or several layers of heavy-duty paper towels. Gather the ends of the towel and twist, squeezing firmly over the sink until no more liquid comes out. This step is absolutely critical; excess water will result in a runny, bland filling. You should be left with a compact, dry ball of spinach.

Step 3: Create the Aromatic Base
In a medium skillet, heat the olive oil over medium heat. Add the finely diced onion and sauté for 4-5 minutes, or until it becomes soft and translucent. Add the minced garlic and the optional red pepper flakes and cook for another minute until fragrant. Be careful not to burn the garlic. Remove the skillet from the heat and let it cool slightly.

Step 4: Mix the Ricotta Filling
In a large mixing bowl, combine the 32 ounces of whole milk ricotta, 1 cup of the shredded mozzarella, ½ cup of the grated Parmesan, and the beaten egg. Add the thoroughly squeezed spinach, the cooled onion and garlic mixture, the Italian seasoning, salt, and black pepper. Mix everything together with a sturdy spoon or spatula until it is well combined. The filling should be thick,  creamy, and flecked with green. Take a moment to taste the filling and adjust the seasoning if necessary. It’s your last chance to perfect the flavor before stuffing the shells.

Step 5: Stuff the Shells
Now for the fun part! Using a small spoon, carefully fill each cooked pasta shell with a generous amount of the ricotta mixture. Don’t be afraid to pack it in. For a quicker and cleaner method, you can transfer the filling to a large piping bag or a zip-top bag with the corner snipped off. This allows you to pipe the filling neatly into each shell.

Step 6: Assemble the Casserole
Preheat your oven to 375°F (190°C). Spread a layer of marinara sauce, about 1 ½ to 2 cups, evenly over the bottom of a 9×13-inch baking dish. This prevents the shells from sticking and creates a flavorful base. Arrange the filled shells in a single layer in the dish, with the open side facing up. Nestle them snugly against each other.

Step 7: Sauce and Bake
Pour the remaining marinara sauce over the top of the stuffed shells, ensuring they are well-covered. This keeps the pasta moist during baking. Sprinkle the remaining 1 cup of shredded mozzarella and ½ cup of grated Parmesan cheese evenly over the entire dish.

Cover the baking dish tightly with aluminum foil. This traps the steam and helps the shells cook through without the cheese on top burning. Bake for 25-30 minutes.

Step 8: Brown the Top and Rest
After 25-30 minutes, carefully remove the aluminum foil. Return the dish to the oven and bake, uncovered, for another 10-15 minutes, or until the sauce is bubbly and the cheese is melted and turning golden brown in spots. For a more browned, bubbly top, you can switch the oven to the broil setting for the last 1-2 minutes, but watch it very carefully to prevent burning.

Once baked, remove the dish from the oven and let it rest for at least 10-15 minutes before serving. This crucial resting period allows the filling and sauce to set, making it much easier to serve intact portions. Garnish with fresh chopped basil or parsley just before bringing it to the table.