Make the 16 homemade crespelles according to the crespelle recipe; let them cool enough to handle.
Preheat the oven to 375°F.
If not already done, thaw the frozen spinach and squeeze it really well to remove excess moisture; chop any large pieces so the spinach is evenly distributed.
In a large bowl, combine 15 oz part-skim ricotta, 1 1/2 cups of the shredded part-skim mozzarella (reserve the remaining 1/2 cup), 1 large egg, the well-squeezed spinach, 1/4 cup grated Parmesan Reggiano, 1/2 teaspoon kosher salt, and black pepper to taste. Stir until the filling is uniform.
Pour 1 cup of the marinara sauce into the bottom of a large baking dish (or divide between two smaller dishes) and spread it into an even layer.
Spoon 1/4 cup of the spinach-ricotta filling onto each crespelle near one edge, roll each crespelle up tightly, and place the rolls seam side down in the sauced baking dish in a single layer.
Pour the remaining 1 1/2 cups marinara evenly over the rolled manicotti, then sprinkle the reserved 1/2 cup shredded mozzarella over the top.
Cover the dish tightly with foil and bake in the preheated 375°F oven for about 25 minutes, until the sauce is hot and bubbling and the cheese is melted.
Remove from the oven, let rest for a few minutes, then serve.