01 - Preheat the oven to 425°F (220°C).
02 - In a large bowl, toss the mushrooms with olive oil, melted butter, garlic, thyme, salt, and pepper until evenly coated. Spread the mushrooms out in a single layer on a baking sheet.
03 - Roast the mushrooms for 20–25 minutes, stirring once halfway, until golden brown and crispy at the edges.
04 - While the mushrooms roast, bring water and milk to a gentle simmer in a large saucepan over medium heat.
05 - Slowly whisk in the cornmeal, stirring constantly to prevent lumps. Reduce heat to low and cook, stirring frequently, until the polenta is thick and creamy, about 20–25 minutes.
06 - Stir in butter, Parmesan cheese, salt, and pepper. Taste and adjust seasoning if needed.
07 - Spoon the creamy polenta into bowls. Top with the roasted mushrooms and sprinkle with chopped parsley and extra Parmesan, if desired. Serve immediately.