01 - Set the oven to 425°F and prepare a large baking sheet.
02 - Toss sliced mushrooms with olive oil, minced garlic, thyme, salt, and pepper; spread evenly on the baking sheet.
03 - Roast mushrooms for 20 to 25 minutes, stirring once until golden brown and tender.
04 - Bring water, whole milk, and salt to a boil in a large saucepan.
05 - Slowly whisk in the cornmeal, reduce the heat to low, and simmer while stirring frequently until the mixture thickens and becomes creamy, about 20 to 25 minutes.
06 - Remove from heat, then stir in butter, Parmesan cheese, and season with black pepper to taste.
07 - Divide polenta into bowls and top with roasted mushrooms, chopped parsley, and additional Parmesan if desired. Serve immediately.