Preheat oven to 375° and grease a large baking dish with olive oil.
Make the noodles: Slice two sides of each zucchini lengthwise to create two flat sides. Using a vegetable peeler, slice each zucchini into thin flat strips, peeling until you reach the center. These are your “noodles.”
Make the filling: In a medium bowl, combine ricotta, Parmesan, egg, 2 tablespoons basil, garlic, and season with salt and pepper.
Assemble the ravioli: Lay two strips of zucchini noodles so that they overlap lengthwise. Lay two more noodles on top, perpendicular to the first strips. You should end up with a “T” shape. Spoon about 1 tablespoon of filling in the center of the zucchini. Bring the ends of the strips together to fold over the center, working one side at a time. Turn ravioli over and place in the baking dish seam-side down. Repeat with remaining zucchini and filling. Pour marinara around zucchini and top ravioli with mozzarella.
Bake until zucchini noodles are “al dente” and the cheese is melty and golden on top, 25 to 30 minutes.
Top with remaining basil and Parmesan before serving.