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1
Break the ziti by hand into 3–5 cm pieces; the fracture should be irregular so that tube lengths vary and nest at different angles. Bring a large pot of well-salted water to a rolling boil. Cook the broken ziti for exactly half the package cooking time. The pasta at this stage must be decidedly underdone — firm to the point of resistance at the centre. Drain and spread on a tray to arrest cooking.
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2
Gently reheat the overnight ragù in a wide pan until it loosens just enough to coat. Transfer the drained ziti to the pan and fold through the ragù until every piece is coated. The sauce must be absorbed into and around the tubes, not pooled at the bottom — this is where the bake begins. Taste now: the ragù should be concentrated and fully seasoned; it will not improve further in the oven.
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3
Spoon half the sauced pasta into a lightly oiled baking dish and level it. Lay the mozzarella slices in a single, even layer across the surface, then scatter half the Parmigiano over them. Add the remaining pasta in an even second layer. This is where the dish lives or dies: the mozzarella must sit in the middle stratum — not distributed throughout every layer and not confined to the top — so that the bottom portion dissolves into the pasta while the upper melt provides the pull.
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4
Mix the remaining Parmigiano with the fine breadcrumbs and scatter the mixture evenly over the top layer. Place a few reserved mozzarella slices across the surface. The top should be uniformly covered — bare patches will not form crust.
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5
Bake at 180 °C in the middle of the oven for 25–30 minutes. In the final 5 minutes, drizzle raw olive oil across the top surface. The dish is done when the breadcrumb-Parmigiano crust is deep golden and set, the mozzarella on top is fully melted and beginning to colour at the edges, and the sides of the dish show the ragù bubbling gently. The top crust must be distinct — not merely warm and yielding.
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6
Remove from the oven and allow to rest for 10 minutes before serving. The interior will firm slightly; the cheese pulls will be more controlled and the portions will hold their shape when lifted. The smell rising from the dish at this point — reduced ragù, browned Parmigiano, warm mozzarella — should be full and round, with no raw-flour note.