Preheat oven to 375 F. Bring a large pot filled with water to a boil.
Cook pasta shells according to manufacturer’s instructions. Drain and rinse with cool water. Line shells on a large baking sheet lined with wax paper or aluminum foil and set aside.
Heat olive oil in a skillet over medium-high heat. Crumble the sausage into the skillet and fry until lightly browned. Using a wooden spoon, break the meat into small bits as it cooks. You don’t want any big chunks.
Combine cooked sausage, ricotta, 1 cup mozzarella, 1/2 cup Parmesan, eggs, parsley, basil, salt and pepper together in a large bowl and mix well.
Pour enough marinara sauce in a 9×13 baking dish to cover the bottom of the dish, about 1 1/2 cups. Fill each pasta shell with about 1 1/2 to 2 tablespoons of the filling, and line the baking dish with the shells. Ladle the rest of the marinara on top. Sprinkle with the rest of the mozzarella and Parmesan, cover with foil and bake until bubbly, about 30-35 minutes. Remove the foil and continue to bake for another 10-15 minutes, until the cheese is gooey and melted and begins to brown.
Remove and let stand about 10 minutes before serving.
Leftovers can be kept in the fridge for up to a week, or frozen for up to 3 months.