Preheat oven to 350 °F.
Bring a large pot of salted water to a boil. Cook pasta shells till al dente, strain, and rinse with cool water.
Meanwhile, in a cast-iron skillet, cook sausage until fully cooked through and browned, about 5 minutes. Use the tip of a spatula to break the sausage into the smallest chunks possible. Season with salt and pepper to taste and remove pan from the heat
Transfer the sausage to a medium bowl with ricotta, basil. garlic, parmesan cheese, and egg. Stir until creamy.
Place 1 cup of marinara in the bottom of the same skillet. Fill each shell to the top with ricotta filling using a spoon. Arrange as tightly as possible.
Top each shell with a teaspoon of marinara. Layer the entire dish with about 1 cup of shredded mozzarella.
Bake for 20-25 minutes until cheese is fully melted and golden. You can finish with a quick broil to give the cheese a little sear if desired. Let cool 5 minutes, top with extra chopped basil and serve.