Bring a large pot of salted water to a boil and cook the jumbo pasta shells according to package instructions, about 9-11 minutes. Drain and set aside.

In a large skillet, heat olive oil over medium heat. Add diced onion and sauté for 3-4 minutes until softened. Add ground beef, breaking it apart, and cook until browned, about 6-8 minutes. Stir in minced garlic for the last minute and season with salt, pepper, dried basil, and oregano. Set aside to cool.

In a medium bowl, combine ricotta cheese, 1 cup of mozzarella cheese, grated parmesan, the egg, and parsley if using. Mix until smooth and season with a pinch of salt and pepper.

Preheat the oven to 350°F (175°C). Spread a layer of marinara sauce in a 9x13-inch baking dish. Stuff each cooked pasta shell with beef filling followed by ricotta mixture, arranging them snugly in the dish.

Pour remaining marinara sauce over the stuffed shells and sprinkle with remaining mozzarella cheese. Cover with foil and bake for 25 minutes. Then, remove foil and bake for an additional 10 minutes until golden and bubbly.

Let cool for a few minutes, garnish with parsley, and serve hot. Enjoy with garlic bread or salad.