Preheat oven to 350°F (175°C).

In a large skillet, cook ground beef, ground pork, and diced onion over medium heat until browned and cooked through. Drain excess grease and let cool slightly.

In a large bowl, combine the cooled meat mixture with eggs, breadcrumbs, salt, pepper, and 1 cup of shredded mozzarella. Mix well to combine.

Cook jumbo pasta shells according to package directions until al dente. Drain and rinse under cool water.

Stuff each cooked shell with the meat mixture.

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

Place stuffed shells meat-side down into the dish.

Pour remaining 2 cups of marinara sauce evenly over the shells.

Sprinkle the remaining 2 cups of shredded mozzarella cheese on top.

Cover with aluminum foil and bake for 45 minutes, until hot and bubbly.

Let rest a few minutes before serving. Enjoy!