1. Bring a large pot of salted water to boil. Cook shells until just tender. Drain and set aside to cool slightly.
2. Preheat oven to 425ºF. Combine ricotta, basil, Parmesan and garlic in a bowl. Season with salt.
3. Spread half of marinara sauce over bottom of a 9-by-13-inch broiler-safe baking dish. Spoon ricotta mixture into shells and place open sides up in baking dish. Cover with remaining marinara, sprinkle with mozzarella and bake 10 to 12 minutes. Turn oven to broil and broil until cheese begins to brown, 2 to 3 minutes. Serve immediately.