In a pot of boiling lightly salted water set over high heat, cook the pasta shells for 10 to 12 minutes, or until al dente. Drain and transfer the shells to a large bowl and toss with a tbsp of olive. Set aside.
In a large saute pan, heat 3 tbsp olive oil over medium heat. Add the chopped onion, bell pepper, and celery, and sauté, stirring frequently, until soft and lightly browned, about 5 minutes. Add the garlic and cook for another minute until aromatic.
Use a small knife to cut the casings off of the chicken sausage links. Use your fingers to crumble the sausage into the skillet with the cooked vegetables. Cook, stirring often, until the sausage is fully cooked, about 8 minutes more.
Stir in the parsley, basil, rosemary, 1 tsp salt, and ½ tsp black pepper. Remove from heat and transfer to a bowl. Place in the fridge to cool for about 15 to 20 minutes.
Preheat the oven to 400°F.
Place the ricotta into a medium-sized bowl and stir in the egg until smooth. Stir in the Parmesan and mozzarella cheeses. Mix until fully blended. Set aside.
Once the meat mixture has cooled somewhat, remove it from the refrigerator and stir in the cheese mixture until fully combined.
Spread about 2 cups of the marinara sauce in the bottom of a 9"x13" baking dish.
Using a spoon, fill the shells with as much stuffing as you can, about 1/4 cup. Place the shells, stuffed side up, on the sauce. Ladle a little more marinara over the tops of the shells. Sprinkle Parmesan all over the tops of the stuffed shells.
Cover the casserole with aluminum foil and bake the shells for about 12 minutes.
Remove the foil and bake for another 10 minutes, or until the sauce is bubbling and the tops of the shells are slightly browned. Serve the shells with the remaining 1/2 cup of grated cheese on the side.