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1
Boil the potatoes whole and unpeeled in well-salted water until completely tender throughout. Drain, peel while still hot, and pass immediately through a potato ricer into a wide bowl. The resulting mass should be loose and steaming, not packed — this heat is the engine of the dish.
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2
Blanch the green beans or turnip tops in a separate pan of boiling salted water until just cooked through but not limp. Drain, squeeze firmly to remove excess water, then roughly chop. The greens must carry no excess moisture; water in the mixture will break the butter emulsion.
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3
If using sage, melt the butter in a small pan and fry the leaves until the butter is fragrant and the leaves are crisp. Remove the sage. Whether plain or sage-perfumed, the butter must be liquid and hot when it meets the potato.
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4
Turn the hot butter over the riced potato. Add the blanched greens and the cubed Casera. Work the mixture with a wooden spoon or a sturdy pestle, trampling and folding rather than beating — this is the 'tarer' technique that names the dish. If using polenta, add it now. This step decides the dish: the potato must remain rough and chunky, with visible pieces of green and melting cheese throughout; a smooth, homogeneous purée is a different preparation entirely and the richness of the butter must read as gloss, not grease.
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5
Taste for salt and adjust. Transfer at once to warmed plates or a warmed serving dish. The taroz stiffens rapidly as the cheese and butter set; it must reach the table immediately.