Place a rack in the center of the oven and preheat to 450°F. Line a rimmed baking sheet with parchment paper. To clean mushrooms, with a soft brush or paper towel, wipe off any dirt from the outsides. Trim off any tough stem ends, then quarter (if some of your mushrooms are super large, cut them into sixths; you want the pieces to be roughly the same size). Transfer to the center of the baking sheet.
Drizzle the mushrooms with the oil, salt, pepper, and 1/2 tablespoon of the vinegar. Toss to coat, then spread into an even layer. Roast mushrooms for 18 to 20 minutes, tossing once halfway through, until they are tender, browned, and caramelized.
Remove from the oven and drizzle the remaining 1/2 tablespoon vinegar over the top. Sprinkle with thyme (if using). Toss to combine and enjoy.
TO STORE: Refrigerate leftover mushrooms in an airtight storage container for up to 5 days.
TO REHEAT: Rewarm mushrooms on a baking sheet in the oven at 350 degrees F. A microwave can also be used, but the mushrooms will be a bit soggy.
TO FREEZE: Freeze leftovers in an airtight, freezer-safe storage container for up to 3 months. Reheat from frozen. Once reheated, frozen roasted mushrooms may be a bit soggy, but they will still have great flavor.