MIX THE POLENTA and the semolina in a measuring cup. In a medium-size heavy saucepan, combine the milk, water, butter, salt, and pepper and bring the mixture to a boil. Reduce the heat a little and, when the liquid is simmering, pour in the polenta and semolina in a very slow, thin stream, whisking constantly in the same direction until all the grains are incorporated and no lumps remain. Reduce the heat to very low. Switch to a wooden paddle and stir thoroughly every 1 or 2 minutes for 15 to 20 minutes, or until the mixture comes away from the sides of the pan and the grains have just begun to soften. Stir in 1/2 cup of the Parmesan and the chopped sage.

RINSE AN 8 x 12-INCH ROASTING PAN with water and shake dry. Pour the polenta mixture into the pan and spread into an even layer just under 1/2 inch thick, smoothing with a spatula repeatedly dipped into very hot water. Cover with a tea towel and allow to rest for at least 1 hour at room temperature and up to 24 hours in the refrigerator.

PREHEAT THE OVEN to 400°F. Lightly oil an oval or rectangular ceramic baking dish, gratin pan, or Pyrex dish.

WITH A 3-INCH ROUND COOKIE CUTTER or glass, cut the polenta into as many circles as possible, then lay them in the baking dish in rows, overlapping each by about half. Sprinkle the remaining 1/4 cup Parmesan over the top and drizzle the cream around the edges and in between the rows.

BAKE FOR 20 TO 25 MINUTES, or until the top is golden brown and the cream is bubbling. Serve from the baking dish set on a heatproof trivet, garnished at the last minute with the whole sage leaves.