For the polenta: Combine milk, 3 cups of water, and salt in a medium saucepan over medium-high heat and bring to a simmer. When it simmers, slowly pour in the polenta and whisk to combine. Partially cover with a lid, reduce heat to low, and cook, whisking vigorously (get all the corners of the pan!) every 5 minutes, until polenta is no longer gritty and looks like creamy oatmeal, about 30 minutes total.
Meanwhile, when the polenta is done cooking, stir in the mascarpone, turn off the heat and cover until ready to use. Taste the polenta and check out the consistency. If you like it looser (as I do), whisk in another 1/4 cup of water as needed to loosen the polenta to the consistency of oatmeal.