Trim the ends off of the zucchini and slice lengthwise on a mandoline to about 1/8-inch thick. Reserve the 16 largest interior slices for this dish and save the rest for another use.
Fill a medium ovenproof skillet with lightly salted water and bring to a simmer over medium-high heat. Add half of the zucchini slices and cook until just pliable, about 45 seconds. Transfer to a paper towel-lined platter to dry. Repeat with the remaining zucchini slices.
Stir the ricotta, basil, oregano, lemon zest and 3/4 cup mozzarella together in a bowl with 1/4 teaspoon salt. Beat the egg well in another bowl, then add 1 tablespoon egg to the ricotta mixture and stir to incorporate. (Discard the remaining egg.)
Pour the water out from the skillet and add the marinara. Bring to a simmer, then remove from the heat.
While the sauce warms, lay the dry zucchini slices out on a work surface. Spoon a scant tablespoon of the ricotta filling on one end of a zucchini slice, then gently roll the zucchini up. Arrange in the skillet seam-side down. Repeat with the remaining zucchini slices and ricotta filling.
Sprinkle with the remaining 1/2 cup mozzarella and bake until the center is bubbling and the cheese is golden in spots, 20 to 30 minutes. Let stand 5 minutes before serving.