Cook the polenta
In a saucepan over high heat, bring 1½ cups (12 fl oz/375 ml) of the milk and 1½ cups water to a boil and add 1 teaspoon salt. Whisk in the polenta. Reduce the heat to low and cook, whisking occasionally, until the polenta is thick and smooth, 3–5 minutes. Whisk the remaining ½ cup (4 fl oz/125 ml) milk into the polenta. Spoon into shallow bowls and top with the asparagus. Top with a dollop of the mascarpone, sprinkle with the Parmesan cheese, and serve.