METHOD

Place oven racks in upper and lower thirds of oven; preheat to 165°C. Combine mushrooms, thyme sprigs and garlic on a large rimmed baking sheet. Season generously with salt and pepper; drizzle with oil. Toss to coat mushrooms, then spread out in an even layer. (Make sure not to crowd the mushrooms on the baking sheet; otherwise, they’ll steam instead of getting crispy.) Transfer to upper rack in oven and let mushrooms roast while you prepare polenta.

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Bring milk to a simmer in a large ovenproof saucepan over medium-high heat. Add butter and a generous pinch of salt and whisk to melt butter. Gradually add polenta, whisking constantly. (Gradually incorporating the polenta into the milk is key to preventing clumps.) Return mixture to a boil, immediately cover pan and transfer to lower rack in oven. Bake polenta, shaking baking sheet with mushrooms occasionally, until polenta is tender, 25–30 minutes.

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Remove polenta from the oven and turn up the oven temperature as high as it will go (but don’t grill). Continue to cook mushrooms until crisp around the edges, 5–10 minutes longer.

Meanwhile, carefully uncover polenta and whisk vigorously, scraping bottom of pan, until polenta is smooth and thick. Gradually add the Parmesan, whisking constantly until melted and incorporated; taste and season with more salt and pepper. Cover and keep warm over low heat while mushrooms finish roasting.

Remove mushrooms from oven; drizzle with vinegar. Toss to coat; let cool slightly.

Divide polenta among bowls and top with mushrooms, thyme leaves, sea salt, and more Parmesan if desired.