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1
Bring the milk to a temperature just below the boil — small bubbles will form at the edges of the pot but the surface must not roll. Keep it at this point throughout the preparation. Cold milk slows the breakdown of the polenta unevenly; boiling milk scorches the base of the pot before the stirring can prevent it.
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2
In a separate small pan, melt half the butter over low heat. Add the sage leaves and allow them to stiffen and release their aroma — thirty seconds is sufficient. Remove the sage and reserve, or leave in the pan to add to the ribòl with the butter. Add the sage-infused butter to the hot milk.
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3
Drop the cold polenta pieces into the hot milk. Begin stirring immediately and without pause with a wooden spoon, working in broad strokes across the base and around the sides of the pot. This is where the dish lives or dies: the polenta catches on the pot base within minutes if the spoon stops moving, and a burnt note, once present, cannot be corrected. Maintain a gentle simmer only — no vigorous boil.
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4
Continue stirring over gentle heat until the polenta pieces dissolve entirely into a smooth, uniform mass. If the ribòl tightens beyond a thick porridge, add a splash of hot milk and stir it through. If it remains too loose, stir on for a further few minutes without adding milk. The ribòl is ready when it coats the back of the wooden spoon in a layer that holds its shape.
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5
Remove the pot from the heat. Stir in the remaining butter in small pieces — the surface should take on a light gloss. Taste and adjust salt. Stir in the grated Parmigiano Reggiano if using. Serve immediately in deep bowls; terracotta retains heat for the duration of eating.