In a large bowl, toss mushrooms with oil, balsamic vinegar (if using), and pepper. Spread on a rimmed baking sheet in a single layer.
Bake for 10 minutes, toss with a spatula or spoon, and bake for 5-10 minutes more or until mushrooms are tender and golden brown. Sprinkle with salt and thyme, and serve warm or at room temperature.
Notes
Most types of mushrooms can be roasted such as baby bella, cremini, morel, shitake, etc.
Make Ahead: Mushrooms can be roasted a day or two ahead and refrigerated. Warm gently in small skillet or oven.
To Freeze: Make a double batch and freeze the extra roasted mushrooms for up to 3 months. If you can, thaw them overnight in the fridge, and reheat gently in a skillet or the oven. If necessary, add a splash of balsamic or broth to rehydrate them.