Lightly oil or spray a 9 x 13-inch baking pan and set aside. Preheat the oven to 350°F (175°C).
Bring a large pot of salted water to a boil. Partially cook the jumbo shells for 4–6 minutes until pliable but not fully tender, then drain in a colander and arrange shells in the prepared pan.
In a medium bowl, combine ricotta, Parmesan, egg, chopped parsley, chopped rosemary, salt, pepper, nutmeg, and crushed red pepper until evenly mixed.
Spoon about 1–2 tablespoons of the ricotta mixture into each shell and place the filled shells in the baking dish.
Spoon marinara sauce on and around the stuffed shells, then sprinkle shredded mozzarella evenly over the top.
Cover the pan tightly with foil and bake at 350°F for about 25 minutes, until bubbly. Remove foil for the last 3–5 minutes if you want a browned cheese top.