Follow these detailed steps carefully to ensure your Spinach Artichoke Stuffed Shells turn out perfectly every single time. The process is broken down into clear stages: preparing the components, assembling the dish, and baking to golden perfection.

Step 1: Cook the Pasta Shells
Bring a large pot of heavily salted water to a rolling boil. Add the entire box of jumbo shells and cook according to the package directions until they are al dente. This is very important; the shells should still have a firm bite, as they will continue to cook in the oven. Cooking them too long will result in mushy, torn shells that are difficult to stuff. Once al dente, carefully drain the shells using a colander and rinse them gently with cool water to stop the cooking process. Lay them out in a single layer on a baking sheet or a clean kitchen towel to prevent them from sticking together while you prepare the filling.

Step 2: Prepare the Spinach and Artichokes
While the pasta is cooking, focus on your key filling ingredients. Place the thawed spinach in the center of a clean kitchen towel or several layers of heavy-duty paper towels. Gather the ends and twist tightly, squeezing out as much excess moisture as you possibly can. This is the most critical step for preventing a watery filling. You’ll be surprised at how much liquid comes out. Set the dry spinach aside. Next, drain the can of artichoke hearts thoroughly. Place them on a cutting board and give them a fine chop. You want small, uniform pieces that will incorporate seamlessly into the cheese mixture.

Step 3: Sauté the Aromatics
In a medium skillet, heat the olive oil over medium heat. Add the finely chopped yellow onion and sauté for 4-5 minutes, or until it becomes soft and translucent. Add the minced garlic and cook for another minute until it is fragrant. Be careful not to let the garlic burn, as it will become bitter. Remove the skillet from the heat and set it aside to cool slightly.

Step 4: Combine the Filling Ingredients
In a large mixing bowl, combine the whole milk ricotta cheese, 1 cup of the shredded mozzarella, 1/2 cup of the grated Parmesan, the beaten egg, the sautéed onion and garlic mixture, dried oregano, salt, and black pepper. Mix everything together with a spatula until it is well combined. Now, gently fold in the squeezed-dry chopped spinach and the chopped artichoke hearts. Continue to mix until the spinach and artichokes are evenly distributed throughout the cheese mixture. Your creamy, flavorful filling is now ready.

Step 5: Assemble the Stuffed Shells
Preheat your oven to 375°F (190°C). Spread about 1 to 1.5 cups of the marinara sauce evenly over the bottom of a 9×13-inch baking dish. This layer of sauce prevents the shells from sticking to the bottom and ensures they stay moist during baking. Now, begin stuffing the shells. Using a small spoon, generously fill each cooked pasta shell with the spinach and artichoke mixture. Don’t be shy with the filling; each shell should be full. As you fill each shell, arrange it snugly in a single layer, open-side up, on top of the sauce in the prepared baking dish.

Step 6: Top and Bake
Once all the shells are filled and arranged in the dish, pour the remaining marinara sauce over and around the shells. You can be artistic here, drizzling it over the top or carefully pouring it in the gaps between the shells. Next, sprinkle the remaining 1 cup of shredded mozzarella and the remaining 1/2 cup of grated Parmesan cheese evenly over the top of the entire dish. This double-cheese topping will create that irresistible golden-brown, bubbly crust.

Step 7: Bake to Perfection
Cover the baking dish loosely with aluminum foil. This prevents the top from browning too quickly before the filling is heated through. Bake in the preheated oven for 25 minutes. After 25 minutes, carefully remove the foil and continue to bake for another 15-20 minutes, or until the sauce is bubbly and the cheese on top is melted, golden brown, and slightly crispy in spots.

Step 8: Rest and Serve
Remove the baking dish from the oven and let it rest on a wire rack for at least 10-15 minutes before serving. This crucial resting period allows the filling and sauce to set, making the shells easier to serve and preventing them from falling apart. Garnish with fresh chopped parsley or basil, if desired, and serve warm.