Preheat the oven to 375 degrees. Cook the pasta al dente, according to package directions. Drain and set aside.
Meanwhile, heat the olive oil in a large skillet over medium-high heat. When the oil begins to shimmer, add the garlic and cook until it begins to brown, about a minute or two. Add the spinach and cook, stirring occasionally, until the leaves begin to wilt but are still bright green, about 3 to 4 minutes. The spinach should be reduced by half. Remove from the heat and let cool.
In a mixing bowl, stir together the spinach, ricotta, mozzarella, Parmesan, egg, basil and salt and pepper until thoroughly combined. Pour 1/2 cup of the marinara sauce into the bottom of a shallow 8-inch by 8-inch baking dish. Stuff each pasta shell with a generous amount of the spinach and ricotta mixture, and place in the baking dish.
Cover with the remaining sauce and bake covered with aluminum foil for 25 minutes. Remove the foil and continue baking until the top begins to brown and the sauce begins the bubble, another 10-15 minutes. Serve warm with a dusting of Parmesan.