In a 3-quart pan over high heat, bring milk to a boil. Reduce heat to a low boil.
Stirring constantly, pour in polenta in a slow, thin stream, pausing occasionally to break up any lumps. Stir in 1/2 teaspoon salt.
Reduce heat and simmer, stirring often, until polenta is thick and creamy to bite and pulls away slightly from sides of pan when stirred, 20 to 40 minutes (see notes); adjust heat as necessary to maintain a simmer.
While polenta cooks, melt butter in a 10- to 12-inch frying pan over medium heat. Stir in mushrooms. Stir often until liquid has evaporated and mushrooms are browned, about 7 minutes.
Stir mushrooms, mascarpone cheese, parmesan cheese, truffle oil, and sherry vinegar into polenta. Season to taste with salt and pepper. Ladle into bowls and drizzle with more truffle oil to taste.