1. Preheat the oven to 400°F. Place mushrooms in a medium bowl and drizzle with olive oil and balsamic vinegar. 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. Add garlic and toss, place back in oven for a minute. Remove and stir in parsley.
2. While the mushrooms are roasting, make the polenta. In medium saucepan, combine milk, water, and salt over medium-high heat and bring to simmer. When simmering, slowly pour in polenta and whisk to combine. Reduce heat to low and partially cover the pan with lid and cook, whisking vigorously every 5 minutes. Be sure to scrape sides of pan. Cook until liquid is absorbed and polenta is creamy, about 30 minutes total. Remove from heat and stir in butter and Parmesan cheese.
3. Divide the polenta into four bowls. Top with roasted mushrooms and garnish with Parmesan cheese, if desired. Serve immediately.