Begin by boiling a large pot of salted water. Cook the jumbo pasta shells according to the package instructions until al dente. Drain the shells and set them aside to cool slightly.
In a skillet over medium heat, add a splash of oil. Sauté the chopped onion until translucent, about 3-4 minutes. Add the minced garlic and cook for another minute until fragrant. Remove from heat and allow to cool slightly.
In a mixing bowl, combine the ricotta cheese, sautéed onion and garlic, 1/2 teaspoon of salt, 1/4 teaspoon of black pepper, dried oregano, egg, Parmesan cheese, and parsley. Mix until well combined.
Preheat your oven to 375°F (190°C). Spread 1 cup of marinara sauce at the bottom of a baking dish. Take each cooked shell and fill it generously with the cheese mixture. Place the stuffed shells in the baking dish, open side up.
Once all the shells are in the dish, pour the remaining marinara sauce over the top. Sprinkle the shredded mozzarella cheese evenly across the shells. Cover the baking dish with aluminum foil and bake for 25 minutes. Remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden.
Remove from the oven and let the dish cool for a few minutes. Garnish with extra parsley before serving. Enjoy your delicious stuffed shells hot and fresh!