Preheat your oven to 375°F and bring a large pot of water to a rolling boil. Add 1 tablespoon of kosher salt and 1 pound of ziti pasta, cooking according to package directions until just al dente, about 8-10 minutes.
While the pasta cooks, prepare the cheese mixture by combining 1 pound of ricotta cheese, 1 large egg, 1 teaspoon black pepper, 1/2 cup grated Parmesan, and 1/4 cup chopped parsley in a medium bowl. Mix until fully incorporated and smooth.
Drain the cooked pasta and return it to the warm pot. Immediately stir in 1 tablespoon of olive oil to prevent sticking, then gently fold in the ricotta mixture and 24 ounces of marinara sauce until the pasta is evenly coated.
Transfer half of the pasta mixture to a 9x13 inch baking dish, spreading it into an even layer. Sprinkle half of the shredded mozzarella over the top. Repeat with the remaining pasta and mozzarella. Pour 1/2 cup of water around the edges of the dish.
Cover the dish tightly with aluminum foil and bake for 20 minutes. Remove the foil and continue baking for another 15-20 minutes, until the cheese is golden brown and bubbly. Let the casserole rest for 10 minutes before serving to allow it to set.
