Step 1: Prepare the Marinara Sauce. Heat olive oil in a large pot or Dutch oven over medium heat. Add chopped onion and sauté until softened, about 5-7 minutes. Add minced garlic and cook for another minute until fragrant.
Step 2: Combine crushed tomatoes, tomato sauce, oregano, basil, and red pepper flakes (if using). Season with salt and pepper. Bring to a simmer, then reduce heat to low, cover, and let it cook for at least 20 minutes while you prepare the shells and filling. The longer it simmers, the richer the flavor.
Step 3: Cook the Jumbo Shells. Bring a large pot of salted water to a rolling boil. Add jumbo pasta shells and cook according to package directions until al dente. Drain well and rinse with cold water to prevent sticking. Lay them out on a baking sheet or clean surface to cool slightly.
Step 4: Prepare the Ricotta Filling. In a large bowl, combine the ricotta cheese, well-drained spinach, 1 cup mozzarella cheese, 1/2 cup Parmesan cheese, and the large egg. Season with salt and pepper to taste. Mix until all ingredients are thoroughly combined.
Step 5: Assemble the Stuffed Shells. Spread about 1.5 cups of the marinara sauce on the bottom of a 9×13 inch baking dish. Using a small spoon or a piping bag (for easier filling), carefully fill each cooked pasta shell with the spinach ricotta mixture. Arrange the filled shells in a single layer over the sauce in the baking dish.
Step 6: Bake the Stuffed Shells. Pour the remaining marinara sauce over the stuffed shells, ensuring they are mostly covered. Sprinkle generously with extra mozzarella and Parmesan cheese.
Step 7: Cover the baking dish with aluminum foil and bake in a preheated oven at 375°F (190°C) for 25 minutes. Remove the foil and bake for an additional 10-15 minutes, or until the cheese is melted, bubbly, and lightly golden.
Step 8: Garnish and Serve. Let the dish rest for 5-10 minutes before serving. Garnish with fresh chopped parsley, if desired. Serve hot and enjoy your delicious spinach ricotta stuffed shells easy recipe!