Peel away all but one of the papery layers from the garlic bulb, then slice off the top centimeter, revealing a cross-section of the cloves.
Place the garlic on a sheet of tinfoil, drizzle with oil and rub it into the bulb. Wrap it up, pop it on a baking tray and roast in the oven for 30 to 35 minutes, removing the foil after 20 minutes, until it’s soft. Leave it to one side to cool a little.
Pour the milk into a large, heavy pan along with 800ml of water and 1 teaspoon of sea salt and bring it to the boil.
Pour in the polenta in a thin stream, whisking all the time so it thickens as you go. Keep whisking for 2 to 3 minutes over a high heat.
Turn down the heat to the lowest setting and cook it for about 40 minutes, or until the polenta starts to come away from the pan, stirring every 5 minutes to stop it sticking.
Melt the butter, then pop half of it into a blender with the basil. Squeeze all of the garlic cloves out of their skins (discarding these) and add them to the basil, along with a good grating of nutmeg.
Blitz to a vibrant green butter, then pour it into a small pan. Warm it through gently over a low heat, then set aside.
When the polenta is ready, finely grate in the Parmesan and stir in the remaining butter. Serve immediately with the basil butter drizzled over the top, finished with an extra grating of Parmesan.