Start by preheating your oven to 350°.
Add olive oil to a pan over medium heat. Crumble and cook italian sausage in pan until browned through. Set aside.
Spray your baking dish with non-stick cooking spray.
Spread 1/3 of the marinara sauce on the bottom of the dish.
In a large bowl, stir together crumbled and cooked sausage, ricotta cheese, parmesan cheese, the egg yolk, Italian herbs and seasoning, garlic powder and salt and pepper until well combined.
Carefully begin to stuff pasta shells with filling (about 1 1/2-2 tablespoons each) and add to the bottom of the baking dish on top of the sauce.
Once all are stuffed and arranged in the dish, pour the rest of the marinara sauce on top and finish by sprinkling cheese over the dish.
Cover and bake at 350° for 25 minutes then remove cover and allow to bake and brown for additional 10 minutes. Let stand 5-10 minutes before serving. Sprinkle with parsley if desired.