Preheat your oven to 375°F (190°C).

Bring a large pot of salted water to a boil and cook the pasta shells al dente according to package directions.

Drain the shells and spread them in a single layer on a baking sheet to cool.

In a large skillet, cook the ground beef, onion, and garlic over medium-high heat until the beef is no longer pink and the onion is translucent, about 8-10 minutes.

Drain any excess grease from the beef mixture and let it cool slightly.

In a mixing bowl, combine ricotta, Parmesan, beaten egg, parsley, Italian seasoning, salt, and pepper.

Fold in the cooked beef mixture and adjust seasoning as needed.

Spread 1 cup of marinara sauce on the bottom of a 9x13-inch baking dish.

Stuff each pasta shell with about 2-3 tablespoons of the filling and arrange them in the baking dish.

Pour the remaining marinara sauce over the shells and top with shredded mozzarella.

Cover the dish with aluminum foil and bake for 25 minutes.

Remove the foil and bake for an additional 10-15 minutes until the cheese is lightly golden.

Let the dish rest for 5-10 minutes before serving.