Preheat the oven to 375°F (190°C).
Cook the jumbo pasta shells according to package instructions until al dente. Drain and set aside.
In a large bowl, mix together the ricotta, mozzarella, Parmesan, cooked sausage, Italian seasoning, and a pinch of salt and pepper.
Spread 1 cup of marinara sauce on the bottom of a baking dish.
Stuff each cooked shell with the cheese and sausage mixture and place them in the baking dish.
Pour the remaining marinara sauce over the stuffed shells.
Sprinkle additional mozzarella cheese on top.
Cover with foil and bake for 25 minutes. Remove the foil and bake for an additional 10-15 minutes until the cheese is bubbly and golden.
Let cool for a few minutes before serving.