Preheat your oven to 375˚F (190˚C). In a large pot, bring salted water to a boil and add the jumbo pasta shells. Cook until al dente, then drain and rinse with cold water.

In a large skillet, add olive oil and sauté the chopped onion until golden. Stir in garlic, salt, pepper, and oregano, cooking until fragrant. Add marinara sauce and let simmer for about 10 minutes.

In a mixing bowl, combine ricotta, 2 cups mozzarella, parmesan, egg, and parsley. Season with salt and pepper, mixing until well blended.

Fill each cooked shell with the cheese mixture, then spread a layer of marinara in a casserole dish. Place the filled shells and drizzle with remaining sauce, topping with leftover mozzarella.

Cover with foil and bake for 30 minutes. Remove foil and broil for an additional 2-4 minutes until cheese is bubbly.

Let cool for a few minutes, then garnish with parsley and serve.