Mandolin the potatoes into thin slices (keep them in water to stop them from browning).
Heat the cream, milk, garlic, black pepper, oregano and salt to a simmer. Add the potatoes in batches and cook until they are pliable, then take them out of the cream and leave them in a separate dish so they don't turn too mushy at this stage.
Place a layer of the potatoes in the bottom of a deep baking dish and top with a few spoons of passata, a good sprinkling of mozzarella and a few pieces of pepperoni. Grate parmesan over the top and then repeat with as many layers as it takes to reach the top of the dish. Over the final layer of potatoes pour all the excess cream and sprinkle with parmesan, then cover and take away to bake for roughly 40 minutes, or until cooked through and bubbling.