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1
Place the rice and cold whole milk in a heavy-based saucepan with the strip of lemon zest. Set over a low heat and bring up slowly, stirring from the start. The milk should approach a bare simmer — never a rolling boil. The surface should tremble, not surge. If it begins to climb the sides of the pan, reduce the heat immediately.
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2
Once the milk is trembling and the rice has begun to absorb, stir frequently and continuously with a wooden spoon, drawing across the base of the pan. This is where the dish lives or dies: vigorous boiling causes the milk to scorch and develop an off-flavour; inattention causes the rice to catch and the bottom to burn. Maintain a steady, gentle simmer for approximately 25–30 minutes, until the grains are fully swollen and tender with no resistance at the centre, and the milk has thickened to a creamy suspension that coats the spoon — pourable, not paste.
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3
Remove the strip of lemon zest. Take the pan off the heat. Season with salt, then add the cold butter cubes and stir vigorously to incorporate. The surface should take on a faint gloss as the butter emulsifies into the hot milk and starch. Add the ground cinnamon or nutmeg and stir once more.
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4
Stir in the grated Trentingrana. Taste. The soup should be gently savoury, milky, and faintly spiced, with the Trentingrana providing a low nutty edge. Ladle immediately into warmed bowls — the soup tightens as it cools. Individual rice grains should be visible and fully tender; the broth around them should be creamy, not watery.