Preheat your oven to 350°F / 175°C.
Cook the jumbo pasta shells according to package directions until al dente.
Drain the shells and immediately rinse with cold water to stop the cooking process.
Drain well again and set aside. 2.
While the pasta cooks, prepare the filling.
In a large bowl, combine the lightly beaten eggs, ricotta cheese, 1 cup / 113g (4 oz) of the mozzarella cheese, 3/4 cup / 85g (3 oz) of the Parmesan cheese, the chopped walnuts, fresh parsley, salt, pepper, and ground nutmeg.
Mix until everything is well combined. 3. Assemble the dish.
Spread 1 cup (240 ml) of the marinara sauce evenly across the bottom of a 3-quart / 9x13-inch rectangular baking dish. Carefully spoon about 1 heaping tablespoon of the ricotta filling into each cooked pasta shell.
Arrange the filled shells in a single layer in the baking dish, filling-side up. 4. Spoon the remaining marinara sauce over the top of the filled shells, covering them as evenly as possible.
Sprinkle the remaining 1/2 cup / 57g (2.01 oz) mozzarella and 1/4 cup / 28g (0.99 oz) Parmesan cheese over the top. 5.
Cover the baking dish with foil and bake for about 45 minutes, or until the sauce is bubbly and the shells are heated through.
Let stand for 5-10 minutes before serving.