Cook the shells according to package directions. Rinse, drain and spread on a sheetpan while you prepare the filling.
Preheat the oven to 350 degrees F.
In a small saute pan, stir together the onion and olive oil. Cook over medium heat until the onion is wilted, about 5 minutes. Transfer to a medium bowl and stir in the ricotta cheese, raisins, olives, and salt and pepper to taste.
Fill each shell with some of the ricotta mixture. Arrange the shells, open-side-up, in a baking dish. Spoon the tomato sauce over the shells and then sprinkle them with Parmesan and mozzarella cheeses. Bake, covered, until bubbling and hot, about 35-40 minutes. Uncover and bake for an additional 5 minutes. Serve hot.