Preheat your oven to 375˚F (190˚C). In a large pot, bring salted water to a boil and cook the jumbo pasta shells according to package instructions until al dente. Drain and rinse with cold water to stop cooking.

In a large skillet over medium heat, add olive oil and sauté the chopped onion until golden (5-7 minutes). Stir in minced garlic, salt, pepper, and oregano, cooking for another minute. Pour in marinara sauce and let simmer for 10 minutes.

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

Fill each cooked shell with the cheese mixture. Spread a layer of marinara sauce in a casserole dish, place filled shells in a single layer, drizzle remaining sauce on top, and sprinkle leftover mozzarella.

Cover the dish with aluminum foil and bake for 30 minutes. Remove foil and broil for 2-4 minutes until cheese is bubbly and golden.

Let the stuffed shells cool for a few minutes, garnish with parsley, and serve.