Preheat the oven to 400 degrees F. Place mushrooms in a medium bowl and drizzle with olive oil and balsamic vinegar. Add in garlic and toss. Season with salt and pepper. Spread mushrooms evenly onto a large greased baking sheet. Roast for 15-20 minutes or until mushrooms are tender, stirring once. Remove from the oven and stir in parsley.
While the mushrooms are roasting, make the polenta. In a medium saucepan, combine milk, water, and salt over medium-high heat and bring to a simmer. When it simmers, slowly pour in the polenta and whisk to combine. Reduce heat to low and partially cover the pan with a lid and cook, whisking vigorously every 5 minutes. Make sure you scrape the sides of the pan. Cook until liquid is absorbed and polenta is creamy, about 30 minutes total. Remove from heat and stir in the butter and Parmesan cheese.
Divide the polenta into four bowls. Top with roasted mushrooms and garnish with Parmesan cheese, if desired. Serve immediately.