Preheat oven to 350 degrees F. Coat the inside of a large oval or 9-inch x 13-inch baking dish with non-stick cooking spray. Set aside.
Bring a large pot of water to a boil and salt generously. Cook pasta in water for 9 minutes. Drain pasta and set aside.
In a large skillet, cook Italian sausage over medium heat until browned, about 8 minutes. Break up the meat as it cooks. Add onion and cook for another 4 minutes until onions have softened.
Drain grease from skillet. Pout 1 jar or marinara into the skillet and stir to combine. Pour meat sauce into the bottom of the baking dish. Spread out into an even layer.
In a medium mixing bowl, stir together ricotta, 1/2 cup Parmesan cheese, Romano cheese, 1 cup mozzarella cheese, garlic, egg, salt, pepper, basil, and parsley.
Fill cooled pasta shells with ricotta mixture. Place in the dish over meat sauce, open side facing down. This will keep the cheese inside and the edges won't get hard.
Spoon the remaining marinara sauce all over the shells.
In a bowl, mix together the remaining 1 cup mozzarella and 1/2 cup parmesan. Sprinkle evenly over shells.
Bake for 40 minutes or until cheese is melted and edges are bubbly. Remove from oven and serve warm.
Notes
You can make this meal vegetarian by omitting the Italian sausage or using a plant-based protein substitute instead.