Preheat oven to 350˚F. Coat the bottom of a large baking dish with 1 tablespoon olive oil. 

Bring a large pot of salted water to a boil. Cook the shells for 2 minutes less than the package directions. Drain the shells, then arrange on a sheet tray to cool while you make the filling. Discard any broken shells. 

In a medium saucepan, heat the remaining 1 tablespoon olive oil over medium-high heat. Add the sausage, breaking it up, and cook until browned, 5 to 8 minutes. Using a slotted spoon, transfer sausage to a small bowl. Add the onion to the pan, season with salt and pepper, and cook until soft, about 5 minutes. Stir in the grated garlic and cook for a minute more. Transfer to the same bowl as the sausage and let cool for about 10 minutes.  

In a large mixing bowl, combine the ricotta, parmesan and half of the shredded mozzarella with the herbs. Once the sausage mixture has cooled slightly, mix into the ricotta mixture. Season to taste with salt and pepper. Mix in the egg.

Coat the bottom of the prepared baking dish with ½ cup sauce. Fill each shell with about 2 tablespoons of the sausage mixture and arrange in the pan with the opening face up. Pour remaining sauce evenly over the shells. 

Bake for 25 minutes, then remove from the oven and top with the remaining mozzarella cheese. Preheat the broiler, then broil the shells for an additional 5 to 10 minutes, keeping an eye on the shells to keep them from burning. Let stand 15 minutes before serving.