Puree the tomatoes in a blender or pass them through a food mill. In a large skillet over medium heat, sauté the onion in the olive oil until golden. Next, add the tomatoes, salt and pepper. Reduce heat and simmer for about 20-30 minutes, until reduced.
Prepare the Filling
In a large bowl, combine the ricotta, mozzarella, Parmigiano, egg, salt, pepper, and parsley. Mix well until creamy.
Prepare the Shells
Bring a large pot of salted water to a boil. Cook the shells until very al dente (half of package’s instructions).
Carefully fill the center of each shell with the ricotta mixture. Place in a baking dish. Cover loosely with foil and bake at 350 degrees for 30 minutes. Serve immediately with a spoonful of tomato sauce and a sprinkle of Parmigiano cheese.