Preheat your oven to 375°F (190°C).

Cook the jumbo pasta shells according to package directions until al dente. Drain well and rinse with cold water to prevent sticking. Set aside.

Heat olive oil in a large skillet over medium-high heat. Add the ground beef (or Italian sausage) and cook, breaking it apart with a spoon, until fully browned. Drain any excess fat.

Add the diced onion to the skillet with the cooked meat and cook for 3-5 minutes, or until softened and translucent.

Stir in the minced garlic and cook for another minute until fragrant.

Add the crushed tomatoes, tomato paste, Italian seasoning, 1/2 tsp salt, and 1/4 tsp black pepper to the skillet. Bring to a simmer, then reduce heat to low and let it gently simmer for 10-15 minutes, stirring occasionally, to allow the flavors to meld.

While the sauce simmers, in a large mixing bowl, combine the ricotta cheese, 1.5 cups of shredded mozzarella cheese, 1/4 cup of grated Parmesan cheese, the large egg, chopped fresh parsley, 1/4 tsp salt, and 1/8 tsp black pepper.

Mix everything thoroughly until well combined.

Spread half of the meat sauce evenly across the bottom of your 9x13 inch baking dish.

Arrange the cooked pasta shells over the layer of meat sauce.

Spoon or dollop the ricotta cheese mixture evenly over the pasta shells.

Top with the remaining meat sauce, spreading it to cover the cheese layer as much as possible.

Cover the baking dish tightly with aluminum foil.

Bake in the preheated oven for 20 minutes.

Remove the foil, then sprinkle the remaining 1/2 cup shredded mozzarella cheese and 1/4 cup grated Parmesan cheese over the top.

Return the dish to the oven, uncovered, and bake for another 15 minutes, or until the casserole is bubbly around the edges and the cheese on top is melted and lightly golden.

Let the casserole rest for 5-10 minutes before serving to allow it to set slightly. Serve hot and enjoy!