01 - Preheat oven to 425°F.
02 - Toss mushrooms with olive oil, garlic, thyme, salt, and pepper in a large bowl. Spread in a single layer on a baking sheet.
03 - Roast mushrooms for 20–25 minutes, stirring once, until golden and tender.
04 - Bring water and milk to a gentle boil in a medium saucepan. Add salt.
05 - Gradually whisk in polenta, stirring constantly to prevent lumps. Reduce heat to low and cook, stirring frequently, for 20–25 minutes until thick and creamy.
06 - Stir in butter, Parmesan, and black pepper. Taste and adjust seasoning as needed.
07 - Divide creamy polenta among bowls. Top with roasted mushrooms and sprinkle with parsley and extra Parmesan if desired. Serve immediately.