01 - Set oven to 425°F and prepare for roasting mushrooms.
02 - Combine sliced mushrooms with olive oil, minced garlic, thyme, salt, and pepper; spread evenly on a large baking sheet.
03 - Roast mushrooms for 20 to 25 minutes, stirring once midway, until golden and tender.
04 - Bring water, whole milk, and salt to a boil in a large saucepan.
05 - Gradually whisk in cornmeal, reduce heat to low, and cook stirring frequently for 20 to 25 minutes until thick and creamy.
06 - Remove from heat and stir in unsalted butter, Parmesan cheese, and freshly ground black pepper to taste.
07 - Ladle polenta into bowls, top with roasted mushrooms, sprinkle chopped parsley, and add extra Parmesan if desired. Serve immediately.