Preheat the oven to 400 degrees F.
Combine the garlic, egg, ricotta, Parmesan, 1/2 cup of the mozzarella, 2 tablespoons of the parsley, a large pinch of salt and a few grinds of pepper in a large bowl. Fold to combine. Transfer the mixture to a resealable plastic bag and snip a 1/4-inch hole in the corner.
Spread 1/2 cup of the marinara sauce in the bottom of a 9-by-13-inch baking dish. Pipe the ricotta mixture into the shells and arrange them opening-side up in the baking dish. Cover with the remaining sauce and 1 1/2 cups water.
Cover the baking dish with foil and bake for 45 minutes. Remove the foil, top with the remaining 1 1/2 cups mozzarella and bake until almost all the liquid has evaporated and the cheese is browned, about 10 minutes more. Let cool 10 minutes. Top with the remaining parsley before serving.
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