Bring a large pot of salted water to a boil. Partially cook the pasta shells in water, 4-6 minutes. Strain the shells and lay in the pan. (See no-boil instructions in notes.)
Mix the ricotta, parmesan, egg, parsley, rosemary, salt, pepper, nutmeg, and crushed red pepper together in a medium bowl. Fill each shell with 1-2 tablespoons of the cheese mixture.
Spoon the sauce on and around the shells. Top with mozzarella cheese. Cover the pan tightly with foil. Bake until bubbly and golden, about 25 minutes. If you want the cheesy top golden, take the foil off the last 3-5 minutes.
No-Boil Pasta Shells: Don't want the extra step of boiling the pasta? Soak the shells in a large boil of hot salted tap water while you preheat the oven and make the ricotta filling. They need to soak for at least 20-25 minutes. Then drain off the water and stuff! When you add the marinara to the baking dish, pour it directly over the top of the shells. Cover and bake as directed.
Homemade Marinara: Want to make your own tomato sauce? Use this recipe!
Crockpot Stuffed Shells: To make this recipe in the slow cooker, boil (or soak) the shells. Then stuff the shells and place them in a slow cooker. Cover with marinara and cheese. Cook on low for 5-7 hours.
Freezing: Stuffed shells are easy to freeze. Make a double batch and wrap some up for next week! Baked from frozen for 50+ minutes.