Bring a large pot of salted water to a boil and gently add the jumbo pasta shells. Cook until just under al dente, about 8 minutes, ensuring they remain firm since they'll bake later. Drain the shells and allow them to cool slightly.

In a skillet over medium heat, warm 1 tablespoon of olive oil. Add the grated zucchini and minced garlic, cooking for about 5–7 minutes until the zucchini is soft and golden.

In a large mixing bowl, combine the sautéed zucchini mixture with ricotta cheese, mozzarella, Parmesan, basil, parsley, beaten egg, salt, and pepper. Mix thoroughly until well blended.

Using a spoon or piping bag, carefully fill each cooked jumbo shell with the prepared filling. Arrange the filled shells on a plate or baking dish for easy transfer.

Spread 1 cup of marinara sauce evenly on the bottom of a baking dish. Place the filled shells on top in a single layer.

Spoon the remaining marinara sauce over the assembled shells and sprinkle the remaining mozzarella cheese atop.

Preheat your oven to 375°F and cover the dish tightly with foil. Bake for 20 minutes, then remove the foil and continue baking for an additional 10–15 minutes.

Once out of the oven, let the shells sit for a few minutes. Garnish with fresh basil and serve hot.