01 - Set the oven temperature to 425°F (220°C).
02 - Combine sliced mushrooms with olive oil, minced garlic, thyme, salt, and pepper on a baking sheet, spreading them evenly.
03 - Place mushrooms in the oven and roast for 20 to 25 minutes, stirring once halfway through, until golden and tender.
04 - In a large saucepan, bring water and whole milk to a boil, then add kosher salt.
05 - Gradually whisk in the cornmeal while reducing heat to low. Stir frequently for 20 to 25 minutes until the mixture is thick and creamy.
06 - Remove from heat, and incorporate unsalted butter, grated Parmesan, and freshly ground black pepper to taste.
07 - Ladle polenta into serving bowls, top with roasted mushrooms and chopped parsley, and add extra Parmesan if desired. Serve immediately.