Cook shells. Follow the directions on your jumbo shells packaging, about 9 min. Drain and allow to cool to the touch.
Cook sausage. While shells are cooking, go ahead and prepare the sausage. Remove the casing and place in a pan on medium-low heat. Break up the sausage into small pieces and cook until no longer pink. Drain fat and set aside.
Preheat oven to 350 degrees. Cover bottom of 12 x 16 pan with a light layer of marinara sauce.
Mix the filling. Put eggs, ricotta, 2 cups mozzarella, parmesan cheese, oregano, garlic powder, parsley, salt, and pepper in a medium bowl and mix well until smooth. Add sausage and stir to combine. If you want to prepare a meatless batch, divide the cheese filling in half, and add the sausage to one half, however, don't add all the sausage, leave about a 1/2 cup leftover.
Insert about 1 Tbsp of filling into each shell, doing your best to fold the outer walls of the shells in—to the help hold the filling in. Place the shells in the pan, it's OK to overlap if needed. Top the shells with remaining marinara. Some people like to smother the shells with a lot of sauce, while others prefer less sauce—it's your call. Cover with tin foil and bake for 30 min.
Once done, remove tin foil and sprinkle with the remaining 1/2 cup of mozzarella cheese and let sit for 5 min. before serving. Serve with parmesan cheese at the table for sprinkling. Caesar salad and fresh bread pair nicely.