Make the tomato sauce: Heat olive oil in a large pot and sauté the onion. Add the tomato purée, season with salt to taste, and sprinkle with oregano. Cook for about 40 minutes.

Prepare the filling: In a large bowl, combine the ricotta, mild cheese, and grated Parmesan. Mash together with a fork. Add the spinach (blanched, drained, and cooled—frozen spinach works fine). Season with salt, a pinch of black pepper, and a little nutmeg.

Bring a large pot of salted water to a boil. Add the jumbo pasta shells and cook for only half the time indicated on the package. Do not stir too much.

Drain the shells and let them cool slightly. Fill each shell with the prepared filling and arrange them side by side in a baking dish.

Cover with tomato sauce, top with pieces of mozzarella and more grated Parmesan. Bake at 430°F (220°C) for 30 minutes. (For a crispier top, switch to convection halfway through baking.)