Preheat your oven to 375°F (190°C).

In a large pot, bring salted water to a rolling boil and add the jumbo pasta shells. Cook until al dente, about 9-12 minutes. Drain and rinse under cold water.

In a large skillet, sauté the diced onion until translucent, then add ground beef and minced garlic. Cook until beef is browned, about 8-10 minutes. Drain excess grease.

In a mixing bowl, combine the beef mixture with ricotta, Parmesan, egg, parsley, Italian seasoning, salt, and pepper.

Spread 1 cup of marinara sauce in a 9x13-inch baking dish.

Stuff each pasta shell with the beef mixture and place them seam side up in the baking dish.

Pour remaining marinara sauce over the shells and sprinkle mozzarella cheese on top.

Cover with foil and bake for 25 minutes. Remove foil and bake for an additional 10-15 minutes until cheese is golden brown.

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