Preheat the oven to 400 degrees Fahrenheit.
In a medium saucepan bring salted water to a boil. Add spinach and cook for 1 minute. Drain and rinse under cold running water.
Squeeze out excess water, chop the spinach, and stir in the ricotta cheese.
Add parsley, lemon zest, and 1 tablespoon of Parmesan into spinach mixture. Season lightly with salt and pepper.
With a mandoline or a sharp knife, cut potatoes lengthwise into ¼-inch-thick slices.
In a large pot of salted water, boil the potato sheets until they begin to soften but before they become too soft to easily handle, about 5 minutes.
Rub butter on the bottom of an 8 x 10-inch glass baking pan.
Add a layer of potatoes to the pan, trimming slices to fit if necessary. Sprinkle lightly with salt and pepper, spread on a thin layer of marinara sauce, then spread on half the ricotta cheese mixture.
Repeat layers, ending with potatoes and a thin final layer of marinara sauce, sprinkled with parmesan.
Bake uncovered for 20 minutes, then cover with aluminum foil and bake until potatoes are cooked through, about 10-20 minutes more.
Let rest for 10 minutes before carving into slices.