Preheat the oven to 375°F and lightly grease a 9×13-inch baking dish.
Bring a large pot of salted water to a boil. Cook the jumbo pasta shells until al dente according to package instructions. Drain and rinse with cool water.
Heat olive oil in a large skillet over medium heat. Add onion and cook for 3–4 minutes until softened.
Add garlic and cook for 30 seconds until fragrant.
Add ground beef, Italian seasoning, salt, and pepper. Cook until browned and fully cooked. Drain excess grease and let cool slightly.
In a large bowl, combine ricotta cheese, 1 cup mozzarella cheese, Parmesan cheese, egg, and parsley.
Stir the cooked beef mixture into the ricotta mixture until fully combined.
Spread 1 cup marinara sauce across the bottom of the prepared baking dish.
Fill each cooked shell with the ricotta beef mixture and arrange them in the baking dish.
Spoon the remaining marinara sauce evenly over the shells.
Sprinkle the remaining mozzarella cheese on top.
Cover with foil and bake for 25 minutes.
Remove the foil and bake for another 10–15 minutes until the cheese is melted and bubbly.
Let rest for 5 minutes before serving.
Notes