Step 1

Preheat oven to 375 degrees. In large pot of boiling water, cook zucchini until just softened, 3 to 4 minutes. Drain and let dry on a paper-towel-lined baking sheet.

Step 2

Stir together ricotta, spinach, 1/4 cup Parmesan, egg, 2 teaspoons salt, and 1/2 teaspoon pepper. Spread 1 cup marinara in a 9-by-13-inch baking dish. Top with 3 noodles, one-quarter the ricotta mixture, one-quarter the zucchini, and 1 cup tomato sauce; repeat three times. Top with remaining 3 noodles, 2 cups marinara, and 1/2 cup mozzarella.

Step 3

Cover with parchment-lined foil and bake 30 minutes. Uncover and sprinkle with remaining 1/2 cup mozzarella and 1/4 cup Parmesan; bake 20 minutes more. Let cool 10 to 15 minutes before serving.