Preheat the oven to 425°F.

In a large bowl, combine the melted butter, oil, garlic, thyme, salt, and pepper. Add the white and cremini mushrooms and toss to coat.

Pour the mushrooms onto a large rimmed baking sheet and roast, shaking occasionally, until shriveled but still really juicy, 35 to 40 minutes (depending on the size of your mushrooms).