Heat the oven and boil the shells: Heat the oven to 375°F. Boil the shells in well-salted water for 8-9 minutes, about 2 minutes shy of the package directions, so they're still firm. Drain, rinse under cool water, and spread them out on a sheet pan or clean towel so they don't stick together.

Mix the three-cheese filling: In a large bowl, stir together the ricotta, 2 cups of the mozzarella, 1/2 cup of the parmesan, the egg, parsley, garlic, Italian seasoning, salt, and pepper until evenly combined.

Sauce the dish: Spread 1 1/2 cups of the marinara across the bottom of a 9x13-inch baking dish in an even layer.

Fill the shells: Spoon about 3 tablespoons of filling into each shell and set them open side up in the dish in a single layer. You should get about 24 filled shells.

Top and cover: Spoon the remaining 1 1/2 cups marinara over the shells, then scatter the remaining 1 cup mozzarella and 1/4 cup parmesan across the top. Cover the dish tightly with foil.

Bake the stuffed shells: Bake covered for 30 minutes, then uncover and bake 10 minutes more, until the sauce bubbles at the edges and the cheese on top is golden in spots. The filling contains a raw egg, so bake until the center of the dish reaches 165°F.

Rest and serve: Let the dish sit for 5 minutes so the filling sets up, then tear fresh basil over the top and serve.